I cut a piece of 1/8" cork to add to my existing rubber gasket to deal with the leaking, and it worked perfectly. I found that the outside shoulder of the cap hit the recess shoulder of the tank before the gasket seated. The extra 1/8" worked perfectly. Maybe try some 1/4" or two layers of 1/8" cork to replace or supplement your rubber gasket.
I have a spare one. There was an updated cap and an older one. In the end I simply pulled out the seal and found for all purposes the plastic on plastic seals well enough.