I wanted to build a sub box for my TSX but didn't want to take up too much space. Using a a Pioneer Premium 8"driver I designed 0.43 cu.ft. box. 13h x 12w x 4.5-9.5 d. Its made with 3/4 MDF, sealed and designed to fit snugly against the rear seat next to the amp (mounted on the seat). I put a 2" lip on the top and one on the bottom to hold it in place. If you drop the seat down you can then take it out. I have a wood shop and all the cuts are very accurate and clean. It needs finished with paint or fabric.

It plays down below 50hz, but I'm not sure of its rolloff. It sounds awesome -- more like a punchy 10 than an 8, however with 2 toddlers in the back seat, its really over-kill. The sub was about $120 and box built by me over a weekend. $50 plus shipping.

I'd also consider selling the Sony amp. Currently it is running the sub and my Alpine 6x9s. It is a Sony XM 504x 4 channel amp BUT one channel doesn't work. So it is now a 50x3 -- or 150x1 / 50x1, but that doesn't make much sense. The other 3 channels have been tested and work great. It has a variable low or high pass, adjustable 40hz boost and accepts speaker leads. $50 plus shipping.

 

closer view

 

my 7 year old sits in the back seat and I have two 12s...

 

Must be nice. I even tried The Wiggles, but still I get "Daddy, that's too loud." Sounds great but for as little as use it, I'd rather have trunk space -- the double stroller won't fit with it in.