If you want to make your own I'd try like a cork/gasket material. Try an auto parts store like NAPA etc..., bring in your cap and see if they can match a rubber washer for it. Probably cost less than $.50. New caps sell for ~$15. I gotta try NAPA to see if I can find the little O'ring seal thingy on the vent screw on top of the cap. I also bought a jiffy model 30 that is probably from the mid 70's.
