When I took the door panels off, just to tighten all the bolts/screws, the plastic sheets were not in too good condition after 30 odd years, so I to was wondering at the time on how to replace them, and what to replace them with.
My youngest son has a friend that works at an automotive glass shop, and he gave me enough plastic to make new ones. I used the old pieces as a pattern, and cut the new plastic (which by the way was a heavier than the original). I used a dum dum strips to adhere the plastic to the door. Worked fine for me. Dum dum is a strip sealant that should be easy to find at any auto parts store.
I asked my sons' friend what the purpose of the plastic sheeting was, and was told it was to keep dust from entering the interior of the car, and to keep moisture out also. It is a moisture barrier.
Have fun, spring is coming and the Challenger is ready to roar.
transman