Question about fuel filler holes

[OhCookie]

Hi Guys, I'm new to the forum. Just bought a 1970 C20 a week ago and need to fix/clean some things. One of the things I want to do is plug up the fuel filler holes. A previous owner changed out the original rear fenders for some reason and the replacements, which don't quite match up, have fuel filler holes. Tanks and other parts are long gone -- looks it was done just to swap the fenders. I'm not going to weld anything in or do any body work. Was thinking of putting in a grommet and gas cap to close them up. Suggestions for plugging the holes?