Any material that is fairly flat, looks like it will hold a staple, and is available with work. Even warped plywood is probably fine because the forms are small enough that the warping may not be noticable.

> Having done remodeling for a number of years, I had the same thought. I'm
> planning on using Blandex (chipboard) for my forms. It's perfectly flat
> (plus, I seem to remember seeing a picture of chipboard forms somewhere).
> I would go with the thicker (not actually 3/4, I think it's 11/16). That
> should also make stapling easier, since 1/2 inch isn't much and there are
> often gaps in the middle of plywood.