I think HDPE would work fine for interior door panels. It's probably as flexible as cardboard, although cardboard will eventually maintain the shape with a bit of moisture. A plus to HDPE would be that it's waterproof.
I had a small leak (that I still need to fix) in my wing window on the right side that soaked the cardboard repro panels I got.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.