Reasonable, were the Model A Lebaron Bonney panels. I used to cut up micro-bus flat panels, and bond the upholstery to patterns cut from them.
The plastic cardboard like they make signs on the side of the road with. After and election u can find them everywhere.
I'm with 35window,I've seen guys use 1/8 pvc,works well and you can bend it with a little heat,I'm working n my interior now and plan on using pvc.
Expanded PVC gets my vote too. If you have a high end lumber yard around they might have it and it's lots cheaper. I think I was paying $80 for 3/4" 4x8 sheets.
Your local upholstery supply house will have waterproof (actually water resistant) panel board in a couple of different sizes. It's reasonable priced and easy to work with. You can cut it with a razor blade and it will hold a staple(s) for attaching your material. PVC might start showing some waves after being exposed to hot and cold, especially the larger panels. Another option is fiberboard, like you get at Lowes/Home Depot, but it's not as resistant to moisture. Just my $.02... Posted from the TJJ App for iPhone & iPad
PVC will actually "melt" and bend with enuff heat in the interior...or as a package tray. There is no such thing as "waterproof" cardboard...'water resistant' yes, but eventually it will warp if it sucks up too much water (rainwater or washing your car). Luan wood is the same as cardboard...it too, will suck up water and warp. Black 1/8" thick upholstery plastic is available from most upholstery supply houses in 4' x 8' sheets ...it won't warp from heat and will not suck up water but it is not cheap...of course, IF you want to re-do your door panels every couple of years cuz they've warped, use something cheap... R-
i use 1/8" white tile board you buy at lowes [it is used to go around a tub or shower] comes in 4x8 sheets
A friend of mine used bathroom paneling he bought on close out at a local big box home improvement store. It had chips and cracks but it was cheap.