Probably you could get the straps for any rack and then twist them with some channel locks so that they become flat. I had to twist the straps that came with my Topeak rack to get it to fit right on my hybrid. The center mount, my guess is used if the bike doesn't have mounts on the side of the seat stays.
Or maybe you could use those metal straps that are used to fasten water heaters to the wall.
Maybe these straps?
http://www.bikebagshop.com/omm-extra...rs-p-1142.html