Not to "poop in anyone's punchbowl" here but...
Won't a rear hitch mounted fuel holder make getting rear ended a little TOO interesting???

As a side note, I fit two of the little 2+ gallon plastic cans on my roof rack last year & hauled them to Moab & back...never used a drop.
Some of those who were there may remember how they looked like they were going to implode in the morning, and in the heat of the afternoon, they looked like they were "inflated".
I still can't believe they sealed that well.