If anyone is still wanting to hang a box from the shelf, this is what I did:
My original post I don't know if it sounds any better than a free-air, but it sure was a lot more work. I also have done some mods to it since the above posts.
1) I enlarged the deck hole to 7". I used a dremel for this, but I went through a whole lot of cutting wheels.
2) Mounted a MDF plate to the deck, similiar to SVTColin's.
I wish I would have made the box 1" shallower, since I didn't need any clearance for the sub magnet. The JL Audio 8W3 doesn't need any space for cooling.
