Some have installed either that sub, or something very similar, behind the passenger seat.
My box is an 8" Kicker, 5 inches thick at the fat end and 3 at the thin end. It sits on the side of the passenger footwell and only slightly restricts side to side leg movement (no length lost).
I'd rather have it hidden, but being black, it blends in well and just hasn't been a priority.
Old pictures...
