Garett, one of my configs had 2 x 100a/h batteries mounted in a ply box on each side of the rear cargo area directly above the wheel well. This gave me 400a/h capacity and it worked well. The batteries were circa 100mm wide and approx 350 high x 400 long - bit sketchy as was a while ago, but another option to save cutting a hole. These were deep cycle agms used as back up power supplies. You can still get them.
This is the style but a bit bigger than the units I used.
Or maybe look at the Optima range - small and powerful?
http://www.ebay.com.au/itm/KICKASS-1...kAAOSw0hRZbvcR