I bought 3 x 5 gallon metal safety cans from lowes. I keep them in a locked tool box and I don't have any problems with fumes because they have a pretty good spring to keep the lid closed on the cans. I find that the aluminum tool box keeps the cans much cooler so there's no swelling.
I didn't think of those, I only have the cheeppoo plastic ones
The transfer tank is a great way to go if one has extra room in the bed of the truck (my bed is 11' so there is no problem there, the other thought is a metal frame mounted on the trailer enclosed with either expanded metal or heavy hog panels so there is full time ventilation. Hinged doors with good locks that make it usable from either side would be a good idea and could also give room for extra like oils, parts and line trimmers as well.
Hank's Lawn Care Service
“Beautifying Lee County One Lawn at a Time”