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A good bit of lateral thinking. If it does not work to your satisfaction, you would still be able to try carbon rods in the channel(s).

The finished neck will have the same overall depth whether it is your one piece design or a neck plus fingerboard. The only way a separate fingerboard will improve stability is if the board is stiffer / more twist resistant than the wood that you would use in the remainder of the neck. I see no (obvious) reasons why your route from the back + skunk stripe approach will not work with a dual action rod. One subtle benefit you can gain from a separate fingerboard is the ability (to some degree) to adjust the relief / fall at the top end of the board by judicious levelling if needed. Despite my physics/engineering former life, I can not reach any conclusions about carbon fibre rods in necks (nor do I have any practical experience with them to go on). It seems to me that it would be all too easy to over stiffen the neck and thus needing the truss rod to add relief ( I unintentionally achieved this state on my last build with a very thick fingerboard and deep chunky neck, no carbon rods). Careful choice of rods seems to be required.

Ben Crowe Crimson guitars video is informative. https://www.youtube.com/watch?v=RHV_LN8F1Qw Others may have different views. IMO one of the key aspects is to use well seasoned wood. Cut, glue, rough shape whatever and then give it a few days before doing more. I have had well seasoned wood move after cutting / thicknessing. This video https://www.youtube.com/watch?v=AU_vdW_HgI0 shows the joys of multi laminate construction aka greased piglet wrestling. Clamps are your best friend, you can never have enough I can see how using a three (or 5) piece laminate neck might aid the truss rod channel and skunk stripe construction by removing the need for a routed channel. Make the centre laminate just slightly wider than the truss rod. Cut out the truss rod slice to the correct depth, length and profile. Keep the 'cut off' slice and cut it down to create the skunk stripe. The ends of the rod where the threads are can often be wider than the main rod and may have dots of weld that need smoothing down. You will have to ponder that a bit. Hopefully the pictures explain.

When you bring the strings up to full tension any remaining play will be pulled solid. This happens even when the holes and bushes are a really good fit. Tuning stability is then not a problem in my experience.

Same problem when I fitted them to my Gibson. I cut strips of plastic milk carton which then filled the gap perfectly. Has worked perfectly for the last 10 years or so. Hipshots sorted the neck dive on the SG bass, they also improved another bass I fitted them to.

I have a similar effect with my G&Ls. TI flats on the SB-1, the mutts nuts, current set 11 years old and going strong. TI flats on the L1505 just seem floppy but I keep persevering and try to use a lighter right hand. Defies all logic.

David Dyke https://www.luthierssupplies.co.uk/ElectricG.htm is frequently mentioned by @Andyjr1515 in threads. I have used 'old' oak and mahogany for necks without problems. The mahogany was from broken down snooker tables, really old, very stable and a pleasure to work with, £4 for a side that would make a body and neck. I wish I had bought much more of it at the time. Maple or Wenge will be fine with flatwound / tapewound strings. Keeping maple clean in the long term might be an issue (or not if you want roadworn). From wikipedia: "The dust produced when cutting or sanding wenge can cause dermatitis similar to the effects of poison ivy and is an irritant to the eyes. The dust also can cause respiratory problems and drowsiness.[citation needed] Splinters are septic." A dual action rod will install in a flat slot. All types of adjuster appear on basses at all price ranges, choose what you like @Andyjr1515 and @Jabba_the_gut are the masters of finish. I find Tru oil or Danish Oil very easy, Osmo Polyx wax was incredibly good on an Oak floor I once owned, have never used it on a guitar build though.

Really like the 'bridge' slots. Knee rests, I remember seeing a rather nifty design for a radical minimal guitar design which I can not find now:(. Think of the following -( rotated 90 degrees.

I have two 'bought' fretless basses and one self made. All have the dots where the lines are (or would be for two of them). The different dot positions do not seem to cause me any difficulty going from fretted to fretless.

The baritone? switch on the EB2 I owned in my youth gave quite a significant bass cut effect. The EB3 in this thread is (Imo) just about the prime period, the mid located pickup hopefully tightening up the 'mud'. A very desirable bass.

The John Birch hyperflux pickup. Allows the pickups to work with different string spacings (bass or guitar) and be adjustable. The JB SG bass I owned in the early 80s had two of them, black, maple neck. It also had some fiendish electronics that I never got to grips with. Had JB convert it back to regular type wiring (they were based in Bromsgrove at that time). The bridge was the same, quite a clever two post design. A well made bass but real bad neck diver

Sometimes called princess tree, empress tree, or foxglove-tree. Apparently native to central and western China and an invasive species in the USA. Also found in Europe including UK. https://en.wikipedia.org/wiki/Paulownia_tomentosa#Europe. I knew it was called empress - used in some G&ls under that name. Possible sources: https://www.ipaulownia.com/en/ (appear to have body blanks (out of stock) and 2 part body timbers) https://www.fyneboatkits.co.uk/supplies/wood/paulownia/ (they appear to do large boards - see bottom of page)

To my take, the ultimate anti twist and relief solution would be two truss rods (I could be totally wrong). Whether this is necessary rather depends on the tensions of the different strings. Choice of woods is almost a religious matter, balsa is probably unsuitable Something hard and wear resistant for the fretboard. Ebony, Katalox, Ipe spring to mind or one of the synthetic / semi synthetic alternatives- rocklite, richlite.

As fitted to all but one of my far too many basses. Longest life so far, 11 years on a regularly used bass :). Long way to go to match the Rotosound trubass strings that did 40+ years on my Eko acoustic bass bought in 1980.

Cylindrical rods will have a tendency to rotate and twist the string as the tension increases. Print cylinders with some flats on the sides so the rotation is stopped by the neighbour.

I bought one of those from a BC member. The wood is really good and lightweight. The fit of the cavity to pickguard screws required work. The finish is ok, at some point I may sand it back and improve the body contours, they are a little crude. I also had to widen the neck pocket.

There are cheapy multimeters that will do inductance, capacitance and resistance LCR, (often do transistor / diode testing as well), they are pretty good value.

The very reason why I build with bolt on necks. It allows me to cover up my lack of skills at your level The build advantage of through necks in this respect is also obvious. Quite why they could not build basses accurately within the desired time/cost constraints in that era is a mystery (or cynically because they could get away with it and did not care). On my 2014 SG bass (surely cnc machined?) it should be an absolute doddle and is really inexcusable. The 'box of bridges' is a classic bodge, reminiscent of British Leyland cars. More time rectifying a car at the end of production than Toyota etc spent building one I seem to remember.

7mm of bone under the string is (probably) more than that under F style and some acoustic bass bridges. No idea why Gibson would fit bone, I can just imagine the amount of breakages when trying to thread it. I believe they also had a spell of fitting nylon saddles to the 3 point bridge. After the correct neck - bridge alignment is achieved, another issue that may crop up is neck pickup alignment. I ended up rotating mine, with the polepieces further away from the bridge, resulting in a reasonably ok alignment. No real discernible tone difference to my untrained ears, instant Andy Fraser/Jack Bruce is readily available.

It is good to see that the neck/bridge alignment is a 'traditional thing' and has carried on from EB0/3 into the later SG bass. The alignment issue is the same (though much more extreme) that my bass has. I can't visualise a solution other than plug and remount the bridge mounting post screws. This then creates the problem of hiding the affected area (Easy with a Hipshot bridge since it will cover it over). @bertbass Interesting insightful observation

A much improved design over the bar/rod with holes extender, avoids the ball ends contacting the body issue. The bridge alignment on my SG bass is less than perfect, if I had not fitted a Hipshot (with sufficient lateral adjustment) it would have ended up being a plug and re-position the posts job. On the plus side the quality of the fretwork is very good, only bettered by the Jon Shuker board I have on a bass he repaired.

I encountered exactly the same problem with my SG bass (same bridge design defect all these years later). I made a 10mm (from memory) aluminium rod spacer. I seem to remember needing to take care with the ball ends and body contact due to the extension back, hole angles and height. It worked perfectly well. Bass now has a Hipshot which is just fine.

You are correct , I was thinking in radii, (R + S/2) and diameters at the same time. With a diagram it became obvious, overall diameters require (D+2S/2) - the string wraps round the post on both sides.

^no need to get out more, plenty of entertainment with engineering mathematics. It is the kind of thing I also do for entertainment, lifelong physicist / engineer here. You have me wondering do you need (b+c)*pi or (b/2+c)*pi, what is the effective diameter causing the length change, however in real world mode, the difference is negligible so I will stop @Smanththread wander. perhaps I need to get out more 🙄