Due to popular demand, I decided to post some pics of my home made tube sub mount.
I used PVC hold down straps, available at any Lows or Home Depot, and then just got some screws and some washers and drilled into the wooden slab that the trunk carpet is stapled to. Hard to explain, but I basically just used screws to bolt the straps down, and wrapped the straps tightly around the sub.
PM me if you want more pics, or have any questions.