I think we can all agree that many materials will make a good seal. The original was made of a rubber extrusion with a round welt showing on the inside and a lip on the bed side. If I were restoring and needed this rubber part, short of having new rubber extruded, I'd look for already extruded rubber parts. One with the large welt or bubble, and one with the lip, and then I'd fit them to the bulkhead with 3M black weatherstrip adhesive. Where I used to restore antique cars, we had books with different rubber extrusions available. I would image Collins Bros has had more of a need for an original rubber for the high dollar restorations that they do and they probably have done all the research to solve this problem. Try calling them and see what they do or have available.