If the tank doesn't have a vapor dome then you will just have to remember to not fill it full. Or only fill it full when you are driving somewhere. The gas comes out of the ground tank at 60 degrees so on a hot day the gas will expand once it gets into the tank. Without an expansion tank the gas will just push out the vent. I ran into this same issue when I had a fuel cell in a race car. The fuel cell was just a box with no expansion area. I filled it up on a cool day and then on the next hot day I had a mess when gas started to push out the vent tube.