RotopaxRotopax makes several size (1 - 4 gallons) gas cans designed for off roading. They definitely won't spill. I carry one on the back of my SXS and it hasn't spilled a drop.
They do store the spout inside the can, so you still have to deal with that. You would need to buy a Rotopax can, Rotopax mount, and some kind of pipe mount to attach the Rotopax mount to your ladder.
Their
RollpaX product is a made specifically to attach to roll bars and such. It would be the cleanest solution, but they're very expensive.