Without a rear tire carrier/bumper, I think putting the spare gas on the roof is a better idea. There are tons of brackets available from many different companies to carry gas on your roof, such as those found at hannibalusa.com.
Whatever you do, don't buy cheap. Get a good Wedco NATO jerry can and a good holder. Expect to pay $100+ for the can and a good bracket. If you are carrying ont he roof then you have to get the NATO can because they can be carried on their sides while the cheap ass Blitz cans leak standing up!
I found a super jerry can holder for my swingaway at 4x4trailers.com, but they don't have one that will work on a roofrack.
Anyhoo, just do a search on Google or wherever for jerry can or jerry can brackets and you will get plenty of stuff to look at.
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ChuckH
"Every morning when I wake up I know it's not going to get any better 'til I go back to sleep again!" Al Bundy