I have made this post a quite few times before, here it goes again.
Most 8" subs will bolt up to the flat area easily. You would have to space a 10" quite far down to get it to fit.
I had a fairly unique setup with my 2 JL 8W3 D4's firing through the rear package shelf. They are mounted in .3 cu ft sealed boxes with fiberfill. As slamming bass was not my goal, going with a small box was fine. JL 8W3 D4 with dual 4 ohm voice coils are wired for a 2 ohm load. Second, build or have built for you sealed boxes that will fit under the shelf. Mine are trapizoidal to match the angles under there. Third (the scary part) cut two 7" holes in the shelf. One between the seatbelt anchor and the trunk hinge, it will just barely fit. The other to the outside of the existing hole (in order to make it symmetrical) Fill in the existing hole(I welded in a patch, but I realize that I am crazy) Cut matching holes in the cardboard and carpet cover. Hang the boxes from the shelf. Wire. Enjoy.
In addition to not rattling the trunk, this setup also preserves cargo space.
Even with two 7" holes in the rear deck I did not have a rattling problem with music, only with certain frequencies on bass test CDs. I also had both my amps suspended from the rear deck, the weight of two sub boxes and the amps may have served to dampen vibration.
cpurser, I do not know if I understand how you plan to use the t-nuts, it doesn't seam secure to me. I used some inserts I picked up at the ACE Hardware. They have a very course thread on the outside that screws into a predrilled hole in your box and accepts a standard machine screw on the inside. If you are careful you can avoid penetrating the box. dnewma04's idea about suspending with screws and fender washers is what I used along with rubber washers for cushioning. I don't know what the total weight of my system is but I used 4 screws for each box and 4 for the amp rack.
