One could take a stock key .... or go to the hardware and get a duplicate plain key cut (I did so on a Toyota key blank), test it for fit, and then ...
... insert it in gas door lock, mark part that is exposed, and remove key. Cut it off a half inch or so above that, and take a piece of wood or plastic, ccut a slot in the base to match stub, attach with JBWeld. The wood or plastic knob can be shaped like anything you want. Another alternative is to solder a flat round or square or triangular or octaganol shaped piece of 1/8" thick brass to the stub right above the portion that extends into the lock, say 1/16" or so. Polish it, engrave it, whatever. It'll be very flat and very unlikely to catch your leg or clothing, and always be there except when you remove it.
Or you could take a solid brass drawer pull knob, cut it in half, slot and silver solder it to that key stub. I said cut in half so that the largest part of the ball shape is near flush with the door surface.