it seems obvious to use jerrycans for fuel. If you buy really cheap ones then of course materials and construction can be suspect. The same applies to any plastic product. Cheap plastic products often use inferior materials sometimes non virgin or regrind plastics . Check out Wiki and the 4wd brigade. For a one of use borrow from a mate.
20l is a PITA to get the filler positioned and safely through the fuel neck flap. I always carry 2 x 10l cans for race days. They sit upright comfortably in the well of an NA boot, not sure about the NB though. They're easy enough to handle for filling. Thin plastic fillers that are stored across the cap and the handle are a weak point. They get damaged by rubbing between cans then they piss down your leg when you go to use them. Doesn't really matter if the cans are metal or plastic, so long as they're decent quality but if it's plastic then I'd go for fillers that are mounted inside the can in future.
If this is for a club run why not just carry an empty mower can as there will always be someone in the group who would be happy to do an emergency run to get fuel if you get stuck.
I'll need to check the driving route. If I know that there'll be a stretch of road between fuel stops that I would be cutting it close then i'll simply fill up the can for that section only.