I used to drive semi-commercially all over the states and the only acceptable plastic gas cans I found at "Canadian Tire" in BC.
Even those cans left in wagon for an 1 hr make you venting the car before entering.
So my vote goes to surplus cans, that can be just mounted on their bracket anywhere you can stand the looks, or if you want to under-mount something, I would check marine tanks.
Depends on your TC, the space on the sides of truck bed makes good storage, if you can find a way to push can there.
Than what is wrong with carrying 5 or more 1 gallons containers?