Body filler in the trunk?

Just my .02. but Im thinking fiberglass with maybe filler on top to smooth it out. Back in the day we would fill trunk holes with fiberglass and mesh. It lasted for a while. I don't remember any cracking. But over time (like 5 years or so) Im sure something might happen to it. if its garage kept might not be an issue with moisture. The seam sealer is also a very good idea. Mopar used seam sealer at the factory. It looks like a wide bead of black sealer. Doesn't have to be perfect cause it wasn't when new. Good luck. :)