If your local hardware store has the rows of boxes from Hillman Fasteners, usually found above the bins of screws, washers, nuts, etc., you'll probably find a nice variety of rubber stoppers. I've been working on my tank and bought two, one for the petcock opening and a wide one for the gas cap opening. The small one was 49 cents, the large one $3. It's kept the acetone in while I was cleaning residual gas varnish and the electrolyte solution when I did the electrolysis treatment (worked great!!) and the glass bead abrasive out while blasting the old paint off.