Mattman has a nice idea, and there even are propane conversion kits for a lot of internal combustion engines. But it's a sort of specialized setup, and IIRC, propane-powered equipment is not as fuel-efficient as when operating on gasoline.
I've used standard three and six gallon tanks for fueling my outboards for years. They ordinarily have a vent screw of some type to account for expansion/contraction, and some small amount of vapor can escape that way. I don't recall it ever being particularly objectionable, because the what comes out dissipates pretty well in the open air. But different folks have different tolerance levels for that.
My tanks would not fit in a 20 mm rocket box, but they probably would go sideways in a 30 mm. rocket box. And of course, they would fit in a typical dry box. But if you are anticipating vapor release, you probably don't want everything in the box smelling like gas.