Frog, I'd be concerned about leakage from such containers. They would have to be vented also I would think. I carry a spare tank that's sold by rocky mtn atv. It is only about 3" thick and mounts flat on the rear rack. Holds 3 or 3 1/2 gal. You can stack up to 800# of junk on top of it without worry. Might be simpler and safer just to fill er up every morning or two.