I havent even tried to come up with specs for a box this big before but figured I try it out and see how close it is. FI recommends 6-8 cubes for BL 18's each. I am running an aq 2200d for now, until I get some extra money and will be running aq 3500d soon. 38" W, 38"H, and 27.5" deep walled behind front seats of 2012 scion xb. Port opening is 7.25" x 36.5" across the top of box, 21" long tuned to 32-33hz. Does this sound correct? Im just pretty much going off what size my old box was internally and changing up my port and dimensions. After I build it in the car, everything will be doubled, but that is not included in those measurements above. Thanks