On my running board I have a clamp that a hold a 2gallon square type of can.
I was given by a friend a suitable fuel can, nice enough and for the purpose intended and clean inside, put some fuel in and realised that there is no small vent hole which means that it will expand and contract with heat.
Would it be sensible to put a very small hole in the brass lid?
I think I have answered my own question and now thinking yes but views are welcome 😀
What is so wonderful about Jeep cans (good ones at least) is that they were designed to tolerate thermal expansion and contraction pressures of ambient temperature changes without venting. The problem with carrying gasoline in a can within a car, especially something like a Jeep or a model T, is that they bounce enough that almost any vent will get splashed onto and then leak all over the place. Even inside a box on the running board, that could become a serious fire hazard.
It is possible, to make a special cap with multiple layers of offset small vents that can minimize such cap spewing when splashed. But they will still also vent vapor. I had to do something similar with my boat-tail roadster T because the tank is turned lengthwise to fit in the back of the body with the filler forward in order to be reached. When going downhill or hitting the brakes, the gasoline all pushed forward and spewed out the cap's vent. Soldering a piece of copper sheeting onto the brass cap, and drilling the small vent holes offset reduced the dripping gasoline mess to an occasionally annoying dribble on the top of the tank.
Drive carefully, and enjoy, W2