Will-
Shamelessly copying the belt slides made by outfits like Kirkpatrick's comes to mind. Lots of cowboy action leather is designed around feeding double barrel shotguns, the designs are fairly simple, and you can pick whichever meets your needs. Just mold the leather over some oak dowels that are the same diameter as your shells. I like dowels, because I can cut them longer than the shells, which makes it easier to work with them. Also, there's no rim to mark the leather.
Cabelas sells some pretty cheap, in their cowboy action leather, too.