I use one of the tractor manual holders mounted under the left bag and a MSR fuel bottle.
Hit the red fuel light a few years ago in Wyoming (never assume a marina will have gas) and had to beg a camper for some gas out of his boat to make it back to civilization.
Last summer, we had 6 bikes on a trip and we had a few detours that took us away from normal gas stops. I knew there was a gas stop at Detroit Lake in Oregon which we were riding around, so no big deal.
Until we passed the gas station and the concrete pads around the pumps had been removed down to the dirt (a foot drop or so). So, no gas.
Three of us were on fumes when we finally got to Sisters Oregon (57 miles), but we all made it. Luckily it was all downhill from the lake to Sisters.
We did have a buddy in his Porsche with us (he had shoulder surgery and was not cleared to ride yet), so he could have gone ahead and bought a gas tank and topped us off if needed.