with very little modification you can fit an intellect 9,6 2800 mah in there.
made by coupling 2 9,6 volts 1400 packs in parallel.
alternativly there is room for 9,6 2000 made from GP 2000 cells. you can check out
www.cheapbatterypacks.com they can make them for you, or you can get the cells individual and make the battery yourself.