gas cap

I created my own locking gas cap by taking out one out the screws around the fill tube and running it from the inside of the trunk out.
To unlock it you will have to open your trunk and unscrew the one screw from the inside
to get your cap off.

I also added a small piece of metal on the inside of the cap to catch on the protruding screw.

This is effective and it keeps the stock look of your car. I doubt even a car show judge would catch it.