Ask and you shall recieve! (Flip Top Gas cap Adapter)
I came up with a thought. (of course I am paranoid)
I wonder if someone likes the flip cap so much that they may want to steal it off the car. So I went to the hardware store and bought one way screws to mount the cap with. fit perfectly work great.
Plus it you want it removed easily you can go into the trunk and grab the screw and back it out from there