"Legality" is going to vary wildly - if you are carrying fuel in the trunk, you're probably pretty restricted, but you may be able to get away with various other mounting options - and I'm sure you can buy them even if you can't run them.

I'd also search out "off road" people and forums, though they may not specialize in your vehicle, they probably know the various rules and regulations.

https://www.zuksoffroadcanada.ca/Gas-Tank-15-Gallon_p_997.ht... does exist, so I'm sure there's things out there. People modifying vehicles for Nunavut or the Australian Outback would be places I'd start.

So do things like : https://masterhitch.com/catalog/truck-suv-accessories/auxili... but I'd be a bit wary of bolting gas tanks to the rear bumper/trailer hitch myself.

And many rules will stop applying once you are "off road" whatever that means in your area.