One of the reasons I got a Yakima Outpost rack was to hand a few Rotopax gas containers off of the left side side-bar. The two photos below reveal what happened when I test-fit the 4.5 Gal. Rotopax on the sidebar of my rack.
Love how the Outpost rack sits well below the roofline. : ) Hate how its diminutive size prohibits a Rotopax mount. : (
Time for Plan-B. I purchased a Roof Rack from a local vendor. I mounted it in an unorthodox way - that is I didn't mount it on top of the crossbars of my roof rack, but rather ran the cross arms of the rack through the side of the black roof rack in order to keep is as low as possible. U-bolts hold the rack in place nicely.
As it turns out, I used all the hardware I intended to use to mount the gas can on the Yakima rack to mount in to the floor of the roof rack. The fit is perfect. I'll add pictures of the fabricated roof mount if anyone wished to see it.
What Crossbars are you running?