Need help with some janky...

[sick472]

The black tar-like stuff inside my doors was likely from the factory and it was tar-like. I applied some heat to it from the outside with a heat gun and was able to scrape the majority of it off. DO NOT add any more heat than needed to make it easy to scrape. Too much heat can do funny things to sheet metal. Note: I did this before doing any prep work to the outside.

Can't help much with the plastic behind the panel other than to suggest something rather stiff like overhead projector plastic. I used some stock-like repo stuff on a F@rd once and it was kinda stiff and about 0.030" thick. Try to situate the sheet so that it directs any water to the slots in the door to get it back in the door cavity. Rope caulk is handy in this application too.