Mmmm, I'd go with the 5.8 x 36 as Greengenes suggested.
Supra does not have the exact specs listed for the 12" profile, but it's pretty much the same surface area as the 10", with a little bit more.
http://rollitup.org/t/heatsinks-for-diy-led-lamps.851907/
I'll show you my math-
5.8" x 36" =...