I have everything pretty much done with my sub box now - I've taken out the spare tire and put in a fiberglass box in it's place. Now that I don't have a spare I keep a bottle of tire foam in the car in case of a flat.
With cover I made for the sub/box:
With the carpet on:
There was a hole in the carpet when I bought the car, I'm assuming the previous owner had a similar setup to mine but with an exposed sub. With this setup I've actually kept about the same weight as stock so no difference in performance. Now I just need to build something for my amps and I'll be done!