Leaking Gas Cap,... Help

[Fordman487]

I have a 1935 Ford 1 1/2 ton pick-up with a fuel tank under the seat. The gas cap leaks when the tank is 1/2 full or more when turning left. I've tried homemade new gaskets for the cap with limited success. Has anyone else had this problem and what's a good remedy? Who sells new caps or cap gaskets? Any tricks out there to try?