Originally Posted by GearNut
Leather makes for a decent gas cap gasket material. Inner tube rubber will eventually swell up to almost twice it's original size.
Edit: Sheet cork, available in hobby stores, also works very well.
Inner tube rubber also dissolves in petroleum products. I have personally done this to prove a point. Leather and cork aren't too bad. But when I built my tank out of a softball bat, I realized I would need to make my own vented gas cap.
I used a large, threaded brass fitting for my bung which had a companion threaded collar. I made a brass disk to close the end of the collar and turn it into a cap. I used a large ball bearing and a hammer to dish the disk a bit. I then drilled a hole for a rivet in the center and a vent hole just off-center, in the disk. I made a Nitril rubber washer to fit inside the disk.
Nitril is ideal because it is NOT dissolved by gasoline. First I soldered the disk into place at the open end of the collar. Then I riveted the washer to the inside of my cap with a small brass washer in between. Air comes in through the vent hole and can get past the Nitril washer, but fumes can't blow out through there - the washer seals against the disk. It is designed, however, to vent from inside if you blow VERY,VERY hard. This is a stopgap against pressure buildup.