Why not look to those who ride MX, ATV, SxS as they carry fuel in the vehicles all of the time. Yes, the Rotopax are nice and lock in but they are pricey once you buy the container and mounting brackets... I have them for on trail use.
Here is was most offroad sports enthusiasts use:
VP Racing Sportsman ContainerAnyone who says a plastic fuel container is no good... hasn't used these containers. The VP Racing containers are reasonably priced ($35), stable, very strong and most importantly, they come with a fuel spout that works... not one of those junk CARB spouts. If you purchase the clear "white" ones, you can easily see the fuel level in the container.