More updates!
For the rear casing I put together a rough shape out of Styrofoam plates.
I then added foil on the inside to get the shape of the walls.
That was then covered with expanding foam. Note: you need to get the stuff that is for cracks, not big spaces. The stuff for cracks is much harder once it dries, and it costs less.
I carved it up after it dried, but it needed some more foam added as it wasn't tall enough on the top. After the 2nd coating of foam dried, I carved and sanded it into this.
The foam part will then be covered with paper mache clay, sanded, then covered with apoxie sculpt. I did some experimenting with the paper mache and apoxie sculpt, and I really think I'll be able to get a nice smooth surface on the casings.
In this picture I have sanded the apoxie sculpt, and covered it with a few layers of primer.
I *was* going to use a pep file, but I didn't find out about them until after I had done this whole process. Since it's for video, and not being worn, I think it will be OK.