Could you possible get some rough measurements? Shouldn't be too hard to squeeze out about 2 ft^3 sealed behind your seat, mounting depth will dictate the most though, and is what will take up the most of your legroom.
I can get some better measurements tommorow but i think i can go about 53 in. long, 8 in. wide at the bottom, im not real sure about the height or width at the top because my middle backrest doesnt fold down into a console and it will get in the way if I go up very high.
with those dimensions and assuming a 4" top depth and 16" height you can get 1.91 ft^3 doesn't look like quite enough mounting depth though for 12's but probably 10's would work. Or if you could increase the height a bit would help, or the top depth would be the most beneficial.