OK google USD SPEAKER WORKS .
Read up on aperiodic membranes and then you will see how i did my sub enclosure some 25 years ago.
2 x 15 in a 0.3 cubic foot box.
IF YOU ARE PREPARED to cut a hole in the floor and reienforce it with a rectangle rim all you need to do is put 2 by 5.5 inch long throw subs into each cubby hole and it will be flush.
Read the white paper and see why its the only way and that about 95% of all winning cars has the amperiodic membrane, and so do i in my competition BMW