Even aside from any potential leak problems you might face, it's just plain unsafe to keep fuel inside of your car like that. Storing fuel inside creates the potential of turning a fender bender into a flaming fatality wreck. Not to mention the fumes...
Thanks Predator for pointing out that some of the external carriers have gas can add-ons. I hadn't noticed the add-ons at first, but double checked after reading your post and found that both JoeHauler and VersaHaul have them available.