Slomofo has some good ideas. I do HVAC engineering for a living and work with vapors in industry on a regular basis. There is going to be vapors if there is fuel (hydrocarbons etc) around. Keeping the fuel tank full will lesson the surface area vaporizing during temp changes and thus lesson the vapor release, so that is a good idea for minimal vapors. If it is as bad in the garage as you say, I personally might invest in an small fan such as you would find in a computer tower and make, or get someone to make me a square to round adapter for each side of the fan to which I would connect that to some vacuum hose or flex hose. Rig up something so the inlet to the hose is over the vent of the tank and the outlet of the hose goes through a wall, window, or anywhere outside. You can order a very inexpensive timer online to turn the fan on and off on an flexible time schedule. probably a minute on and a minute off. Run the little fan and blow the fumes outside. WW Grainger or KELE in Memphis have tons of timers. The condom or balloon idea may work just fine too. I can't see any reason why other than I'd change them out pretty regular if the vapors eat away the rubber in the balloon.