Bob Drake locking gas cap

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Anybody else ever have trouble putting a Bob Drake locking gas cap on a '32 Vintique gas tank? I can't get it to work.

It fits and locks but you can't get the key out because the lock hits the inside lip of the filler neck. Is there a simple fix for this like a thinner gasket or something?

I could grind down the inside of the filler neck with a Dremel but I really don't want to risk getting shavings in the tank.