The guys on an off-road forum put me on these when I mentioned I go places with long distances between gas stations.
Rotopax sells a tiny one gallon can that rocks, and has been tried and tested true not to leak under the most extreme ADV bikes and Jeep conditions. Some even have them mounted upside down.
On long trips I always set a bag on my bag seat. Next long trip i might set one of these "1 gallon" tanks under the bag.

This is how small it is on a 250 enduro

For those without a top box trunk there's a mount to put it on our rear rack:



Here it is on a small rear rack:
