I made my own out of anodised aluminium, mostly 3 x 50 & 3 x 40mm flat bar, & 40 x 20 x 3mm angle under the containers, with 316 stainless steel Allen M6 bolts, & some M4 bolts, a few countersunk bolts in the marine hinges & stainless latches. Everything tapped & also used Nyloc nuts.

20 x 3mm 316 stainless steel flat bar in captive channel in roof rack which is tapped for M6 bolts.

4 container holder, in pairs for expansion, are 10 litres. 2 container holder in wheel are 5 litres. 

I have some 20 litre containers, but they are too heavy for me these days. 10 litre containers on & off the roof is easy. Also being lighter you will actually use to rotate fuel. 20 litres you tend to avoid rotating fuel into the car as they are too heavy.

The hardest part of the project was thinking it out to fit in with the other stuff. Building it was easy, & it has worked really well, especially on corrugated roads. Can't think of anything that I would change.

I had a roof platform & replaced it with 4 cross bars, net weight saving was 16kg. Aluminium has a specific gravity of 2.7, slight variation for various alloys. 

All the off the shelf jerry can holders are really heavy, because they are steel.

Fuel we get into the car asap. Solar cells, there is basically a shelf to place them overnight so we can sleep in the car.