Ok,
So I went with the Odyssey P680. Of course, it is much smaller than the battery box in the sponson, so I fabbed up an aluminum bracket, lined it on one side with .125 closed cell foam. There was so much space left that I decided to stick the MSD in there too.
I know, don't put the MSD in closed spaces...well, its not an entirely closed space, and I doubt it gets any hotter in there than many places in the engine compartment. If it doesn't work out, I'll pull it out, and there will be only four quarter inch holes hidden away as evidence that it was ever there. Worth a try, I don't like the idea of having it out in the open.