Jim, the best replacement for the rubber under the straps that I have found is glass setting tape. It's the rubber tape that you soak with oil to secure the door window glass into the metal frame.
It is available in different widths and thicknesses. If you search "glass setting tape" on eBay, you will get several hits.
Local glass shops that specialize in car window replacements usually carry it in large rolls and will sometimes sell you the amount that you need.
I get it wide enough to overhang each side of the metal strap by about 1/4" because that's how it looks on the stock installations. I think the overhang helps to protect the tank from the sharp edges of the metal straps.
This is the stuff that I use for in-cab tanks. It works so well and looks so much like the original material that I sometimes wonder if this is what the factory used.
Good luck,
Matt
