Aaron Armstrong made them. They are just a couple of MM5 shells, because the one which was in there already was an MM5. The J ones should be in where they need to be. The P will be slightly spaced out and it is an experiment but it will work, even if it is not "pure P". I have put an Armstrong P5 into my MTD Kingston which is in the MM spot, and in the thick of a live band I have not felt that the position being a few mm to the left of where it "should" be is an issue and it still does "P".
I was originally going to put one big cover in with multiple magnet options in there but because the original rout was MM5 with ears it would have to have been the size of a bus to cover all the holes. If you are putting a new block of wood in the middle, then you could put a large one piece cover in - like the monsters you were considering earlier, just with your own configuration in there.