Rotopax are good if you need to mount them funny - in a funny position, a funny location, etc. Otherwise, your best bet is a good 'ol 5 gallon Jerry Can. I was running two of the same 5 gallon cans as Celt (the shorter, fatter style jerry cans) and loved them. They work well with a factory roof rack.
With my aftermarket rack, I have two 2 gallon Rotopax cans mounted to the front where the basket drops down. Nice thing about them is that they lay flat and don't stick up above the side rails of the rack. The bad thing about them is that they cost a fortune, they only hold 4 gallons, and they don't stand up on their own to be filled. Drives me nuts! On longer trips (Mojave Road, Death Valley, etc), I still have to bring one of my 5 gallon cans.
Those 5 gallon cans are sold at ORW. They call them "used oil cans," because they don't have the stupid EPA nozzles required to be gas cans. That makes them even better.
I think I paid about $80 for the two of them together. Worth every penny.