Ok, this is new to me. Living up here in the frozen north we probably don't have any fuel vapour (oops vapor according to my American spellcheck

). We just buy frozen cubes of gas and throw them in the heated tank to melt. JK
Do all your pumps have the sealing rubber? I assume from the pics that you have to hold the rubber piece compressed to allow fuel flow? So you slide the seal back, install the tool in a slot on the nozzle and then can hold it up to allow seeing the filling of the tank?
The tool you have made is very simple and compact although I don't know if I would want to keep it in my wallet after it is exposed to the gas.

Good luck with the sales. I would be interested to hear where all this type of nozzle is in use. I suspect that in the not too distant future we will be all using some form of the same filler nozzle.