I have a Rotopax 1 gallon gas container that I attached to my AltRider rack for my trip to AK last year.
I filled it up at the start of the Dalton Highway and carried the gas all around Canada and Alaska then all the way back home (ended up burning it in my lawnmower
).
I never did have a need for the extra gallon, although it is cheap insurance.
I might be willing to sell mine, with the mount. It shows some wear from the trip, but is in good condition.
PM if your are interested.