After several weeks of inactivity due to work commitments, I finally finished cleaning out the inside of the back. The plaster mold worked very well. It held the back firmly in place, allowing fairly aggressive wood removal. Even without clamping the frame, just laying it on a rubber mat, the mass kept movement to a minimum. However, clamping the entire assembly to the workbench was even better.
After removing most of the excess wood with the larger finger plane, I cleaned out the remaining irregularities with scrapers and a medium finger plane. Checking the plate thickness with a graduating caliper from Atlantic Violin Supplies, I found that everything was almost exactly right. There were several inconsistent areas, which were easy to address with the smallest finger plane and scraper. I did a quick tap-tone test. I'm not very good at judging tap tones, but I think it's coming in at about a G. Once the remaining cleanup is done along the perimeter, it should come down to about F, which is where I want it.
I tried fitting the top plate into the plaster mold. It's not a very good fit. So I'll have to destroy this one (so I can reuse the frame) and create another for the top plate.