I've always had great luck with cork gasket material. In the "old" days, 40's - 50's, cork was used on gas tank caps almost exclusively. I have cork gasket sheets in my gasket material bin and use it quite often when I need to make a gasket that thickness is not an issue.
I would try a local auto parts store and ask for cork gasket material. Usually in one foot square sheets. It will be around .080" (2mm) thick and holds its shape quite well. I would cut it to fit tightly inside the cap and then use the bolt with double nuts to hold it in place in the cap. The double nuts will allow you to snug it up and then lock the nuts together.
I'm sure there are other materials that would work, but this just jumps at me, make it cork...
Dan
