Is there a slim battery i can get for my truck?

I want to add a second battery to my bronco 2 to put in the rear beside my subwoofer enclosure. My battery in the front is not enough for the truck and stereo but i need to get a really slim battery to fit back there as my box is 5 1/2 cubic feet and takes up almost all the back end. Ps i dont want any suggestions on bigger alternator or capacitor i simply want an answer to my posted question thanks.