For those of you interested in covers for your fuel caps, I have a set that I won as a door prize at one MAPA convention.
I believe are no longer available. However, I think I can give you everything you need to manufacture a set.
They are made up of three components.
First is the covers themselves, which are perfectly circular and approximately 6 inches in diameter. The size must be large enough to cover an area around the gas cap with enough extra for the sealing material. Mine are made of some kind of plastic, which is semi-rigid, but a non-bendable aluminum or even a clear plastic like plexiglas or lexan would work as well. Mine are about 1/16th inch thick, but again, I don't see why something a little thicker wouldn't work as well.
Second is the sealing material. It is merely a strip of foam rubber like one would buy to fit around a door for weather stripping. It is sold at hardware stores here and has a self adhesive on one side; you merely pull off a paper strip and adhere it to the outer edge of the cover. This weather stripping is about 3/16ths inch thick and 3/8ths inch wide. Neither of these dimensions are critical, but it must be bendable enough to go around the cover without buckling or wrinkling.
Lastly, the fastener. Mine use what I call "super velcro", but is more technically called "recloseable fastener". One type is "3M Dual Lock Reclosable Fasteners", but there are others. (Ordinary Velcro is not strong enough). On mine, the fastener is about 3/8th inch square. (Too small and it won't hold; too large and you couldn't pull them off.) One side of the fastener is permanently glued to the cover and one half to the lever of the gas cap.
Depending on whether the cover flexes a little or not, you may have to shim the fastenner away from the cover. Ideally, when the covers are installed, the foam is compressed a little to keep water from running under it.
Let me know if you have trouble finding the fastener material. I either have some left from a project, or can locate some.
Also, let me know if the above description doesn't make sense some way. I'll be glad to help if I can.
Don