Cut four square pieces of 1/2-inch MDF, measuring 12 by 12 inches. These are the front, back, and two side panels of the box.
Cut two rectangular pieces of MDF, measuring 11 by 12 inches. These are the top and bottom panels of the box.
Assemble the top, bottom and side panels of the box to form a hollow box, measuring 12 by 12 inches on the edges. Screw the panels together with 1-1/2-inch wood screws, spaced every 3 inches.
Cut an opening for your subwoofer in the front panel, using a jig saw. Use the subwoofer as a template to size the opening.
Screw the front and back panels onto the box, using 1-1/2-inch wood screws, spaced every three inches.
Drill a 1/4-inch opening in the center bottom of the back panel with a drill.
Slide a length of speaker wire into the box through the opening in the back. Pull the wire through the front opening and attach it to the subwoofer's terminals.
Place the subwoofer into the opening in the front and screw it into place with 1/4-inch wood screws.