I don't know if this will help, but I use 1/4" plywood (5 ply) from scrap crates used to ship solar panels. You can also use plywood from art/craft stores like AC Moore. Before I had the 1/4" plywood scrap, I used plywood from AC Moore and glued two 1/8" sheets together with wood glue and then cut out. Some folks may argue that you've got to use aircraft grade plywood, but for the foam planes, the craft stuff will work fine. So far I've had great experience just drilling 1/32" holes and screw mounting the motor to the plywood.
Cutting plywood is difficult to do by hand, it is designed to be strong, so I would probably not attempt doing so with a straight bladed tool... I value my digits.
I use Elmer's glue to attach the paper template to the plywood and then cut on the scroll saw. Add a little water and it peels off afterwards. If you don't have a scroll saw, a jig saw, even a small hand saw will work. My firewalls are rectangular glued to foam support. Never had an issue with FT planes.