+ you can easily swap back to arbitrary storage once you are finished with your rare 2000 km journey.
For all it's issues, petroleum is extremely convenient in that it can be transferred and contained in such a low tech way with (relative) safety.
The only downside (shared by my initial thought) is that they aren’t collapsible. So that volume is committed even 1/4 way through my trip
I’m curious if these are legally allowed in Canada. If so they seem decent!
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.
Your alternative is of course to fill some plastic bags from your local grocery store like this [1] lady did.
[1] https://twitter.com/FiendishlyYours/status/13919904872220016...