Since your goal is to carry gasoline for emergencies rather than to extend your range on a regular basis, you might consider a 2 gal. Rotopax can instead of the typical 20l/5 gal. fuel container - less bulk and less than 1/2 the weight when full.
At 18.5" x 13.5" x 3", a Rotopax can is small enough to put behind the rear seat which would eliminate the need for a permanently mounted carrier or one that straps to the spare tire.
Although I do not use Rotopax containers for my jeep, I use them for other vehicles and have for several years. When the caps are properly tightened they do not leak or vent fumes, not even when laid on their side and stored in the hot sun. [Tips: don't fill to the brim, leave some air space, and buy a spout for the Rotopax water cans to use in place of the CARB spout supplied with the fuel cans.]
Mine, stored flat on the roof of my Xterra, photo taken on a Baja California beach last Christmas season:
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If those don't leak or have fumes, that would be an easy way to go for sure. I could easily just strap them on my wooden cover that goes over my back seat area. That was the main reason I wanted something for outside the Jeep, don't want to have to smell gas fumes. I'll look into those cans. Thanks.