Like Bill said, just add plastic on the edge! What could be easier? You DO have plastic sheet, yes? That is just one supply every builder should have... styrene sheet in various thicknesses. I cut out a trunk on a '57 Ford using the back of a #11 blade, added the inner lip to the opening on the body for the trunk lid to rest on (one more reason to have plastic sheet on hand), then placed the trunk back in position... yep... gap is too big.
Cut a piece of plastic, glue it to the trunk lid edge. When cured, I used the moto tool to take the excess 99% down, and finished it off with a sanding stick. I think it takes more work for me to type this out than to actually fill the gap!
EDIT: BTW, in theory, on a four- sided body panel, you should need to add sheet stock to two of the four edges. I think adding plastic to all four edges would make the fit too tight.