Well I gave cutting the fillers a shot. I made a template of the 3/8 rabbet and then used that to mark the filler.
I bandsawed close to the mark and cleaned it up with a drum sander in the drill press.
Test fit - not too bad. It looks like after gluing in the fillers I can flush sand them on the top and back. Then I could band saw close to the body and trim with a flush cut bit in the router table.
(A little pressure helps close thing up)
I didn't take a lot of time with this. All of it was done in about an hour. I wasn't really in a hurry I just worked at a steady pace without taking a lot of time on either step. I wanted to see if this idea was going to work
So it wasn't really all that hard. It looks like with some good wood and more time spent fitting the parts this just might work. The fits really aren’t too bad for the little effort.