Your local hardware store should have a selection of large rubber corks- one should be close enough in size to use or modify a little, so it goes inside the neck allowing the cap to stay on and look original. You could put a smiley face sticker on the plug to further discourage or amuse enthusiastic gas-pumpers.
Dave
Well, my time went so quickly,
I went lickety-splitly
Out to my ol' 55.
As I pulled away slowly,
Feelin' so holy,
God knows, I was feelin' alive..... Tom Waits, Ol' 55
| 
