Rotopax provide some really cool mounting options, so if that is the priority, by all means, go with them.
Most of my complaints are directed at the fuel containers. The water containers are easy to use, so I'll keep those, but still, the water containers (at least the ones I bought) are quite small for the price. I'm going to replace my fuel containers.
Complaints:
1 - Each of the ones I bought (2 water containers and 2 petrol containers) is two gallons. Price per gallon they are the most expensive option available. In retrospect, I would have gone with Scepter for both fuel and water (see the OD Green container below). They come in 2.5 gallons and 5 gallons - actually 10 and 20 liters - but as an American I don't really know what a liter is
I'm fairly certain that Scepter are certified - at least here in the U.S. (thousands of these are used by our military). Another option if these aren't certified in the U.K. would be the classic NATO container (here in the U.S. a company called Wavian makes these for civilian purchase - see the red container below).
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2 - Its kind of hard to pour the petrol into the petrol-tank of the car, and I always end up with petrol on my hands. A funnel helps getting the gas into the truck, but for some reason, its harder to get a good seal on fuel canisters - but its relatively easy to get a good seal with the water containers. A little pump would make this easier and much cleaner.
3 - The problem getting a good seal is magnified in really cold weather (-15 to -20 degrees Centigrade). The caps that screw on and off the Rotopax are hard to undo and tighten in really cold temps. We only do two winter trips per year where we need to carry fuel, but its still an annoyance.
4 - The petrol containers swell in the sun. This seems to be worse when they are empty and contain petrol residue that expands as a gas in the heat.
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Bottom line: I feel like there are other options that would work for my needs that are both cheaper and better.