Gas cap leak

This is probably a dumb question, but here goes anyway in the hope that I'm just missing something obvious. I have a leak from my gas cap which is driving me insane. It happens when the tank is any more than probably 2/3 full, and only filling a Roadster tank half full to make sure I don't get gas-legged leaves me with a bit of a limited range.
I've tried sanding the top of the filler neck so everything's level. I then tried building up the filler neck a bit with JB Weld and resanding. I've replaced the rubber seal on the stock cap, and eventually just replaced the whole damn cap, seal and all. Each time it makes it a little better but never actually eliminates it. Anybody suggestions?

I had a similar problem on an Interstate tank. I put on a new cap, filed the filler neck, no joy. I finally realized the fuel was coming from between the filler neck and the tank opening. I cleaned the inside of the tank where the neck is soldered (brazed?) with acetone and used JB weld. It seemed to stop the problem, but I repainted a different tank and side covers and relegated that one to a space on the wall.

I'm not saying this is where your leak is, but it's another place to look.

Check the cap rubber seal is fully down on the neck seat.
If not carefully tap the latch down on the tank neck. Get a helper to support the neck with a big as pos hammer head. Or buy a new cap.

Cash