tricks for gas cap?

I have the original 76 KZ750 fuel tank on my bike. The open/close lock works, but after I close it and lock it, after about a mile of riding the vibration has unlocked it and it bounces like crazy. Has anyone else had this problem and if so what did you do to fix it?

had this problem
took the cap completely off the tank
took a good look at it
found it oxidized with a broken spring
took it apart cleaned and oiled it
put in a new spring
used a toilet gasket from home hardware (had to cut ID to correct size)
for a new cap gasket
never happened again