[vettestr]

Hello Dog,
Glad the rubber expanding plug worked to seal the major fumes for you. An insert or new base adapter with a gas cap is the ultimate fix and is also available with a locking gas cap. Short of replacing the base adapter to accept the cap the plug does work and is a fix for 3 bucks.

The base does need to be sealed to the body, a gasket can be made from Buna sheet rubber and simply cut one out using the base as a pattern. RTV is the short cut fix and the Ultimate Grey is good with gas. RTV works but is tough on paint, it is under the cap so not seen I agree but buna gasket is a better fix I believe.

Try putting a drip pan full of charcoal briquettes in the trunk for a few days with plug installed. It may pull the fumes out enough to make OK as they absorb the fumes. Hope it works but shampoo carpet if not.
Jeff C