You could purchase a kit like one from Summit or Jegs and put it into a vented box. You could also use a sealed Optima battery and leave it exposed, not the greateat idea though. Unless you have to move the battery I wouldn't recomend it because like someone said the trunk is small enough.
I saw it done on an 04 with a twin turbo set up. It looked really weird, but they also removed the spare tire and mounted a fuel cell in that area so the old tank was out and they had alot more trunk space.
Yeah sounds like a good idea, but the trunk is super tiny. i don't even have a trunk now. I put two mtx 12" 9500 subs and an mtx Thunder Amp 92001. No more room. Of course putting the gas tank, and the battery in the same area does worry me a tiny bit.