In the time before children, I owned a ski boat. The fuel cap was like yours now Large, but there was a vent on the back of the boat that expelled any excess fuel into the lake but I guess it allowed it to breathe. Boat gas caps shouldn't leak for the reverse reasons we are dealing with (water).
I look at my lawnmowers and the gas caps have holes in the top, with a foam filter then a piece of plastic. That keeps the fuel in and the dirt out but lets the air in. Just like the HappyTimes tank, enough vibraton and fuel will come out of the cap.
Perhaps if I can find some foam filter and make it fit around the breather part of the cap, it will "catch" fuel splashing or evaporating. Won't likely help in the case of a bike tipped over.