The tank I got came with a sheet of self-adhesive 1/16" thick neoprene which you could cut as needed (according to the instruction sheet that came in the box). Very impressive product and less costly than many of the other options out there. If your outside rubber grommet doesn't seal quite right, use some 3M 08509 Auto Bedding and Glazing Compound. It stays flexible until a week after forever.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end