If it's not structural I guess it really doesn't matter what ya use as a form. As Rich mentioned I've seen cardboard, etc.
Personally I've used composite(s) on my last two rebuilds. Yeah I know it's a lil more expensive BUT. It'll never ever absorb water and/or rot. And you can tool it with a razor knife'n a piece'a sandpaper. Plus you don't have to seal it, or worry about whether the glass is gonna stick.