I made seals for the fuel tank fillers out of 3/8 inch thick closed cell neoprene. To make the inner hole, I found some steel pipe 3/16 inch narrower than the fuel tank filler neck, and cut out the doughnut hole by heating the pipe with a propane torch and melting it out. I used scissors to trim the outer diameter and attached the seals with 3M weatherstrip adhesive.
