I built a prototype battery bracket out of some 20 gauge stainless I had laying around. I intended it to be the actual bracket, but as these things go, after I had the bracket built and tried to install it I came up with a slightly better idea.
I am using a Powermaster D680. This battery has the same specs as the Odyssey PC680, but is significantly less expensive ($80 vice the $110 for the Odyssey). The Odyssey has a very good reputation and has actually achieve FAA certification for use in light aircraft with 360 cubic inch air cooled engines. As long as the D680 lives up to its specs it should work fine.
The best thing about this batter is that it can easily fit in the engine bay and slide through the side of the frame without removing the headers.