3/4 MDF and a roll of that carpeting stuff. You can get it at Wal-Mart or an autoparts store.
Spray adhesive and solvent for the carpeting.
I built a couple boxes for some 8 inch woofers that I was using for midbass. I made them square because it was easier and I didn't really care if they were cool looking, I just wanted them to be simple and look nice. I bought the MDF at Lowes and had dimensions for the squares and had them cut them for me. I used a jig saw to cut out the opening for the speaker. Drilled some holes and then screwed the pieces together. I sealed the seams with some sort caulk that was appropriate for that use. I drilled a hole for the speaker wire, ran the wire through, then I sealed that.
Cut the carpet up and glued the pieces to the box. I used the solvent to clean the corners so that you couldn't tell where the seams in the carpet was.