1971 Demon 340 T/A build help

It's not rubber, it's a hard material (Delrin???) and is, I believe, produced on a 3D printer. Lots of folks here have used these adapters with good results. Drop damraider a pm and he can provide details. Installation takes some work because the gas filler tube mount is different from year to year. My tube and flange installs from the inside of the trunk (isn't that how yours is???) and the new, longer screws go through the gas cap, through the adapter and through the quarter panel holes and screw into the filler neck flange like the originals.