1963-1966 Early A-body Gas Cap

All you need to do is add a tab on the inside of the cap that lines up with one of the screws that holds the fill tube to the body of the car.

Then you replace that screw with a longer one and insert it from the inside of the trunk so that the tab keeps the cap from rotating when the screw is tightened.

to unlock you open the trunk and loosen the screw. the cap should rotate freely