I had a box of modelmaking plaster left over from a previous project, so I made a frame for the violin back and mixed up a batch of the plaster. The mold is made from a couple of boards also left over from another project. I laid a layer of plastic (an opened-up shopping bag) over the mold, poured in most of the plaster, laid another sheet of plastic over the plaster, then pressed in the back plate. I had to push the plate down quite firmly, as the plaster has the consistency of thick pancake batter.
I've since trimmed the plastic back and taped the edges down. I tried to lift the entire plaster block out of the frame, but it wasn't moving. So I'll have to make another frame for the top. However, I did do a few trial passes with the large finger plane, and the mold holds the plate in place very nicely. I think this will work out fine.