well what it is is the actual cargo area, there is a foldable floor and underneath you have about 4-5inches of space to put cleaning supplies, etc. (or perfect spot for amps), and then underneath that part on the right is where the stock battery is
so no, there is not enough room for another battery IMO...now...ive seen other magnums modded out to make room for more but the most common ive seen is this setup
there are cubby pockets on the right and left of the car in the rear....if you really want a second battery, put it in that right cubby, you will have to fab up some sort of enclosure to make it look nice but thats up to u