Interesting question - I'll be looking at the answers. The originals in my '57 Chevy were a fiberboard material, like you'd use for a kid's science fair exhibit. I hadn't thought of the Lexan concept, but it sounds good.

The "Christmas tree" clips that I've seen have generally been in newer cars, with formed plastic backing, and once the're in, you can forget taking them off without replacing at least the clips and, in the case of a couple of Fords I've got, the door panels.

The Shelby has metal spring clips that fit into holes on the door interior, and they're available from Mustangs Unlimited, American Mustang, Year One, and so on. They'll fit on the fiberboard backing, since that's what's behind all the fancy formed foam on the Shelby door panels. For the Chevy, unless I hear of something else here, I'll probably go to the upholstery supply shop (I think I saw one in Marysville, CA), and get some weather resistent backer, and use the clips.

But there's bound to be a better suggestion show up here...