IMHO, you should not stow/store outboard gasoline on the boat either above or below deck when you’re off the boat. But if you’re going to, lash the fuel tank to the stern rail so if it leaks, the gas will hopefully run out the gunnel to “wherever.”
This is also the best place to stow outboard fuel when underway, which I think is your question.
The only good way to dispose of unused gasoline following a cruise that I’ve found is to pour/transfer it into the auto’s tank and start with fresh gasoline each cruise. I have one of those cheapo transfer pumps for that. Mixed gasoline for 2-stroke outboards is more of a problem. I basically do what Benny described when on a cruise. Keep fresh gasoline unmixed until ready to use it; put in only the amount needed as best I can estimate as I use it. A few ounces left over is not a big threat, so you may be able to treat the container as empty at that point. Old gasoline with EtOH is not good for the outboards anyway. So, that’s another reason not to keep and try to stow/store unused gasoline for long.
BTW. If you want to put the gasoline-filled tank somewhere on the boat where it will ride during a cruise until you need to get it out for use, that’s STOWING. If you want to put it somewhere to rest between cruises rather than to discard it (the gasoline) that would be STORING. Interchanging the terms leads to some variation in responses.