[QUOTE=MadScience;186239]As Carringb alluded to, you want to either not have vapor build up, or no ignition source. If you work to minimize the chance of ether, you're probably OK.
Eliminating the vapor is definately the best choice. Should vapor build up, say on a hot day where the box is an oven, ventilation perhaps in the form of a louvered vent in the back, top and bottom will help. Should enough vapor escape to bring the atmosphere somewhere between the upper explosion level and the lower explosion level, any ignition source (perhaps a spark from the door latch?) will set it off. Having said that, I suspect it would be very unlikely. Just keep the cans sealed and enjoy having spare fuel.