On a trip across Wisconsin I set the fuel cap on top of the pump and didn't realize I forgot it until I looked in my mirror and saw my keys still in the fuel door.

Course I had gone thirty miles by then.
That's the second or third one I've lost.

Problem is the DW really gets a kick out of it.
So I went to the auto parts store and hedging my bets bought two.
When I got home I drilled a hole in the protrusion on the top and ran a lanyard through it securing the other end around the fuel inlet neck.
Course since I bought a spare I probably won't forget it again.
Hate to keep entertaining the DW like this.