TTT got it right..

when i used mine, i made it out of 3/4" mdf (multi-density fiber board (i think)), cut all the sides, predrilled every hole like he said.. so it wouldnt split.. the hardest part was to get the hole cut for the actual sub.. we used a jigsaw to cut it out, before we screwed all the sides together. i also used caulk to seal all the joints on the inside, so no air would leak. obviously, put the whole thing together, except the back and caulk the inside of it.. then put the back on and caulk that from the big hole in the front..

i also cut a hole in the back of the box and put a wire terminal thing off of one of my old surround sound speakers, so i could connect it to the box all nice.

if you know what you are doing with wood.. its not hard. i spent probably 25-30 bucks on the whole thing. i didnt carpet it either.. its just chillin my trunk, no one sees it