55.2-59 1957 Chevy Pickup inside door panel fastener issue

[Donowprod]

While attempting to take the door panel off to replace my window regulator I sheared off two of the retaining screw heads. In an attempt to drill and use a screw extractor, the nut popped off the backside of the door. It seems like it was held on by some sort of flange. What have others done to fix this. Is there replacements that can be installed or plastic plug / nuts like those used in drywall be used. Any help would be appreciated!
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