go to home depot, buy a sheet of 3/4 mdf, have home depot ppl cut it for you, screw and wood glue. Have someone with a jig saw or just take it to some car audio shop pay 5-10 bucks to have em cut the subwoofer hole for you. Put everything together add more wood glue then caulk the edges, get some cheap 99 cent spray paint and your DONE. less then 60 dollars in supplies.
You can also get a custom built one by a local shop. That's what I got done and, in fact, it's getting installed this weekend. Coincidentally, I'm heading straight to SD once it's installed. I can refer you to my shop, though it's in LA area. Do some searching in your area, but I personally only go to places that someone else has vouched for.