Don't fill the tank and use fuel proof paint. Carry a few paper towles with you when you ride and wipe away the seepage. (discard the used paper towles in a government approved container so the tree huggers don't come after you.)
The leakage is from the vent. Not a lot you can do about it. Double gaskets won't help. The tank has to be vented to the atmosphere and gasoline will find its way out of any opening, which the vent is. Deal with it. It's just a fact of HT/kit part ownership.