Why not an aftermarket spare fuel tank? With a switch to convert between your regular tank and the backup?

One thing I've noticed about late-model vehicles...there's plenty of "exterior" cubby space (not in the passenger compartment, but under the shell) where a smallish (5 gallons or less) tank would fit.
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Member of the toughest, meanest, deadliest, most unrelenting -- and ablest -- form of life in this section of space, a critter that can be killed but can't be tamed. --Robert A. Heinlein