Just thought I'd share this idea,
I wanted a couple of subs in my truck, but didnt like any of the options out there. All the boxes Ive seen either take up too much space and you cant fold the back seat down, or they are tiny low profile boxes that dont look like they wouldn't put out very much. What I ended up doing was mounting the speaker box in the bed of my truck underneath my tool box, and then porting it into the cab behind the passengers seat. The box itself is made of 3/4" MDF and is a 3 chamber box with each chamber being equal size. I made the box so it would barely slide under the tool box, it is about 28 inches wide and 13 inches deep. I mounted 2 10" subs in the box, one in the left chamber and one in the right chamber both firing into the center chamber. The center chamber is sealed and the left and right chambers are ported from the side facing the cab of the truck with the port holes entering the chamber in about the center of each chamber. For the ports I used 4" pvc. Now to deal with the flex between the bed and the cab I bought a rubber flexible pvs couplings from the hardware store. This is basically an 8" length rubber tubing with a 4" ID. When I cut the port holes in the box I cut them to the OD of the rubber tubing (PVC coupling). I then used a razor knife and cut the rubber tubing into 1" rings, you will need 4 of them. Before assembling the speaker box I placed the side peice with the port holes up to the the back of the bed where the box was going to be placed and then traced the port holes onto the back of the bed with a sharpie. I then used an air body saw (harbor freight $20) to cut out the holes in the back of the bed (you will use alot of blades and the air saw blades can get expensive, just cut off lengths of a hacksaw blade they will fit in the harbor freight body saw). I then transfered the pattern to the the cab by carefully drilling many holes around the perimeter of the port holes I had just cut into in the bed into the cab. Now I had a perferated pattern of holes to follow on the inside of the cab. I then followed the circle of holes and cut the port holes into the cab from the inside of the cab. I assembled the box, coated the outside with Thompson water seal and then black outdoor paint. I then epoxied one of the rubber 1" wide rings into each port hole. I mounted the box in place and used "L" brackets to mount it to the bottom of the bed, and then ran some screws from inside the bottom of the tool box into the top of the speaker box to secure it. Then from inside the cab I inserted the 2 4" PVC ports each approx 7" long through the holes in the cab and into the rubber lined ports of the speaker box. I then inserted the last two rubber rings between the PVC tubes and the holes I cut in the cab. You dont want rubber rings around the holes in the bed you only want rubber rings in the speaker box and the cab, the PVC ports need to pass through the bed with plenty of clearance for body flex. I then used black silicone to seal around the rubber rings in the cab. I mounted a small sub amp under the passenger seat, drilled a small hole through the cab and bed and ran the speaker wires through it, and then sealed the rest of the small hole with black silicone. The only down side to this whole thing is perminently cutting holes in your truck, but I dont plan on ever getting rid of mine so Im not too worried about it. Its been there for about 6 months and I hanvt had any problems with leaking or with water damage to the box. It sounds really good, most people who get in my truck look around and ask where the subs are? Hope sombody finds this idea useful.