"I have an old locking gas cap with a not-so-tight fit"
Seems it would be a whole lot easier to change the gas cap. On real hot days I'll get a slight gas smell outside the cab. Especially if the windows have been up and its been sitting a while in the sun where the cab temp is pushing 130+. But if your getting real strong smells its time to change that cap. A new non-locking cap will run you less than $10 at your local parts store.
