I'm planning on making my own, too ... eventually.
When you say cardboard ... what exactly do you mean? Certainly, regular cardboard doesn't have the firmness or consistancy you need ... but I have some stuff that I think would work well -- it's about 1/16th of an inch thick and is a sort of waxed cardboard; very sturdy, yet flexible.
What may be best, and I've heard guys using it with good success, is plastic (ABS?) that comes in 4' x 8' sheets. I haven't checked it out yet, but if it's plyable enough, it sounds like a good choice. They say you can shape it with a heat gun and get good results, too.
