From that picture of yours, id just remount the 13W7 in a new box, facing the cabin under the strut bar, you wont have the air space issues then.
If you still think the W7 is too big, why not get a smaller W7, i have an 8W7 and its great, but you could go for a 10/12 W7, or if you want 2 subs, why not 2 10W6v2's?