Using the pass thru is pretty much out of the question for 15's...honestly i would just make a standard rectangle box...36 inches wide and rougly 16 tall and probably about 12 deep.
That would equate to about 1.5 cf per sub....then throw some fiber fill in there to get more density inside the box.

Play around with this and you can figure out how much air space you can get with what dimensions.

Rockford Fosgate® - Woofer BoxWizard