styrofoam is ok to use. The only bad part about it is when shaping it if you trim to much its kind of hard to 'put it back'.
What I usually do is get small wooden dowel (sp) rods from a hobby store (Michaels, home depot, etc...) and glue together a small skeleton frame. Then I staple and/or glue the fiberglass to the frame and make it. Then I just pull or cut the wooden dowels out. It's worked pretty good so far, and since this really wont be visable and doesnt need to be extra perfect it might work for you.