Billy, I think it sounds pretty unlikely, it's just a matter of -what-if-
To be honest, I'm more concerned about some arcing event while it's in the garage, which is adjacent to my children's rooms.

I can't decide if I want to seal up the fuel compartment and ventilate it, or hang either the battery, or the fuel, off the side.

The only reason I asked if the vapor floats or sinks is to consider it for venting.

Sands, at least around here, home depot seems to run out, then get a batch in, if you have several home depots around check them all; I've seen them recently. I actually went with a different style of tool box, simply because it can fit under the bed cover in my pick-up without being disassembled; but I'm pillaging all the ideas from this thread.