What's it called, and where do you get it?

Definitely Luan. If you can't find it easily, just cut up some old bedroom doors for it. Same stuff.

 

This I think is the best material to use, make certain it's the water proof version

 

The coated cardbord is available at upholstery suppliers but I use door skins from Lowes or Home Depot.

 

Remember... Door windows leak, even with new rubber. Get something WATER PROOF!

 

Masonite and you want it thin. if your going to spray glue some fabric on you MUST HAVE a smooth non porus side or else the glue will soak in and the fabric will un-mount......do NOT use plywood or the stuff you use for sub floors. also you can use drop cloth plastic as a barrier between the door panel and the door to make it waterproof.

 

Home Depot sells a 1/8" hardboard of some sort, dark brown, same as pegboard but no holes, for like $7.50 for a 4x8 sheet. Probably not waterproof, but if you're on a budget you can do what GM did and use it anyways with some plastic sheeting on the inside of it. I think it would be fine for something like a '50 Chevy that has big, flat door panels and a lot of metal inner door to keep the moisture away from it.

I bought a leftover s**** piece cheap to use to repair an old folding table (hey, new ones at Wal-mart are $40, old frame was free, leftover piece of board was all of a dollar) and figure I'll see how hard it is to work with then.