My inner door skin is Al and is 19 years old and hasn't cracked yet. I would get 032 Al as rgatijnet1 suggested. Use the old skin as a template to cut the new one. As an alternate material you can use a sheet of reinforced FG and epoxy. .030-.050 would work. It is much stronger than unreinforced plastic(pure resin). Unreinforced plastics expand much more in the heat than FG laminates and may be expanding with no where to go being trapped by the return flange. When installing the new inner door allow for some clearance for expansion, maybe 1/16 of an in.