The Stag Arms has a 1/2-36 thread muzzle brake they use for the 6.8 SPC. It resembles the Miculek brake. The best thing is it's only $15.99 here:
http://www.riflegear.com/p-214-stag-...ator-68mm.aspx
Since it's made for a .277 bullet I'd suggest running a 21/64" (0.3281) drill though it before attmepting to shoot .308 bullets.
Eduted 8/22/2008
I just measured an unmodified 1/2-36 brake from Stag Arms and it's clear bore diameter is 0.404" diameter. That's usable for 6.8, 308, or even 9mm without drrilling.