1963 gas cap?
This one is a NOS factory replacement in stainless steel (with chromed pot metal handle) instead of the assembly line painted one, but structure is the same. (I have a painted one somewhere, but can't find it at the moment.) There's a rubber seal between the brass flange and the center hub thingy. It's just loose, and can be (carefully) pulled off over the hub, so fabricating something that will work and seal shouldn't be too hard.
This one is a NOS factory replacement in stainless steel (with chromed pot metal handle) instead of the assembly line painted one, but structure is the same. (I have a painted one somewhere, but can't find it at the moment.) There's a rubber seal between the brass flange and the center hub thingy. It's just loose, and can be (carefully) pulled off over the hub, so fabricating something that will work and seal shouldn't be too hard.