Gas tank venting

Short version: for those bikes which don't vent thru the cap, how are the tanks vented and is it possible to adapt that to a non-vented tank?

Long version: when I got the bike, the lock in the original gas cap was beyond repair so I bought a $20 Chinese replacement cap on flea-bay that actually looked and fit great. Unfortunately it was unvented, and after 10 minutes I'd get a strong enough vacuum to prevent the bike from running. Once I figured this out, I frankensteined the lock and actuating mechanism from the new cap into the old cap. It sort of works, but now it takes 8 or 10 tries to actually get it open. Evidently Frankenstein isn't so healthy. So it seems like my choices are either find an old gas cap on ebay that's in decent shape AND vented, or figure out some other way to vent the tank.

Anyone ever retrofit a vent into a non-vented tank? My motocross bike has a vent spout right in the middle of the cap that you stick a hose on run wherever. No issues with its gas evaporating or smelling up the garage.

Related topic rant: this paranoia in our country that a drop of gas will evaporate and kill a tree or something is really starting to pi$$ me off. The last plastic gas can I bought has some stupid spring loaded valve thingy built into the spout that is impossible to operator one-handed, and every time I use it I wind up spilling gas everywhere. Someone will have to explain to me how that is better.