the sub needs to be sealed to the box, and the box needs to be sealed too. Unless you lay fiberglass in the well i cant see you being able to seal it, thus meaning it'll sound pants. The wheel well prob isnt very big either (you'll be lucky to get 1cuft), meaning your restricted to small subs (a week 10" would prob be ok)
If you must do it, the only possible way is to make a false floor from 12mm mdf, and cut out the hole for the sub above the well. you'd then have to make a liner for the well from fibreglass, and bond it to the underside of the false floor. You'd have to soundproof between the fibreglass and the well to stop vibrations.
I really cant see the point in doing this. You could face it upside down, but again i cant see the point. Just build a regular box!
