My 2 cents on the topic - I've made all 4 door panels and one cargo panel (jack storage on the pass side) with plywood and transferred complete panel upholstery from original ones (detailed description in
this and
this posts).
Original cardboard panels were warped (especially drivers door). Unlike Michel's mine didn't have any foam padding on the panel (not counting tiny foam pieces on the clips) - given that everything looked untouched inside the door, I'd say that by 88 they didn't have any additional sound insulation other than cardboard itself. Maybe they did on earlier years, I don't know.
I didn't have marine grade plywood, so I used regular birch (1/4 or 1/8 thickness, can't remember) and put couple layers of waterproof/sealer on them. Initially I made holes for factory-style metal clips, but later changed all of them too this style
That required grafting plastic inserts into plywood
I also covered whole panel with CCF sound insulation, that helped tremendously with road noise (I also put kilmat on all doors, roof, quarter panels and almost the whole floor)
Overall I'm very satisfied with how it turned out, especially given that it was my first time doing anything like this and I was figuring out the final design as I was going. If I had to do it again I'd never go the plywood route - too much work for waterproofing, tracing, cutting, fitting, etc. Plus what Tim mentioned about safety and splinters from plywood.
I'd get ABS panels (BJ's or from
men in black member on this forum, I believe BJ's actually sells his panels), drill holes for plastic clips I've mentioned above, transfer upholstery on them and cover inside with foam. For foam, you want to use
closed cell foam (CCF), as it won't absorb/hold any amount of water, unlike open cell.
Another thing with ABS panel is that you'll probably need to heat the up and bend a little to account for that bend in the door metal
With plywood I had to cut top 2 inches of thick plywood off and replace that section with thinner material (I used 1/8 marker board, which is suboptimal, but works) - you can see white section with no foam in 3rd pic. Otherwise you might end up with very noticeable gap between door and panel, more details on that in
this post