Regular carry of gas should be in a metal can. Plastic is simply not durable enough for the job (even though many use them, potential problems still exist even if someone hasn't experienced them).
I like metal "Jerry" style (WWII German Military style); they handle very easily, are simple to secure-store (chain, for example) and are durable. New or surplus, your choice. Probably limited to IMP 5 Gal [US 6 Gal / 20~25 litres] but that's your goal with two smaller cans.
If you *really* need small cans due to some space / storage / handling limits, I personally would then look at aluminum fuel bottles designed for camping and gas stoves. A 1 litre size should be easy to find, perhaps 4 of them (slightly larger than 1 USG) would be good enough for you.
If they seem too expensive, buy one or two and keep adding at that same rate until you have the 4 or 8 litre capacity you need. They are legal (red anodized, typically) fuel containers, which solves some potential issues you might have with non-gas cans (legally, the filling station is supposed to prohibit you from filling it if it's not a "gas" can or the right colour).
Would fit upright and nicely in typical car carrying stuff from bags (with a bungee cord) to boxes.
I've carried JetB in one for literally 20 years on a daily basis, an MRI 1 litre one (use for emergency fire starter) and that includes in the passenger compartment of aircraft on a regular basis (DeHaviland Otter, mostly). Pilot is absolutely fine with it, and just so you know he's not cavalier about safety, he would not let me carry Bear Spray until I found a crash-proof container to hold it. "If that stuff gets out somehow, the plane crashes because I won't be able to fly it" or something like that. Note that Bear Spray is many times more concentrated than what the police etc can legally carry.