I've just read this thread and here's another method for you.
I have a video tutorial by Dan Erlewine which stresses the need for a full body shim rather than just a Fender-style one at one end of the neck pocket. Dan is the man when it comes to guitar repair and works for Stewart-Macdonald guitar shop supply.
The reason given for this is that the Fender one leaves an air space which can take in moisture eventually causing a "kick-up" or "rising tongue". This is where neck swells and the last few frets are angled uphill rather than downhill or flat, so strings constantly buzz against the upper frets. The bass would then need a partial re-fret and maybe some wood taken off the fingerboard.
Ideally a shim should taper from the height needed at the butt end of the neck pocket to zero at the top. This prevents the air-space problem and the possible consequences. If you have some woodwork tools and skills you could trace the neck pocket, lay out the holes for the neck screws, then grab a block a wood cut it the same size as the neck pocket and drill the holes for the neck screws. Bandsaw from your measurement to zero, clean it up and you're good to go.
However, I don't have either the machinery or the skills
The alternative is to lay out a ladder of 3 or 4 narrow strips gradually getting thinner towards the top of the neck pocket. Put a ruler over the top to check that it goes from your measurement to zero. I've used cardboard in different thickness (box to business card to cornflakes pack) to get this.
I know this goes against what has already been taught here, but it's essentially the same thing with 2 or 3 gradually thinning shims added. I've done to a couple of instruments and it's only a little more work than the Fender shim.
I'd rather spend a little more time trying to find a couple of thinner materials than have a potentially expensive repair further down the line.