[tireburner163]

Alpine recomends a 0.85 to 1.5 cubic foot box for a Alpine Type-S 12" sub.

I would just build a single box, but with a dividing peice in the middle, so it's like 2 boxes.

Make it out of 3/4" MDF, use liquid nails and screws to hold it together (remember to pre-drill the holes or the MDF will split) and seal all the inside edges with silicone so it's air tight (and be sure to let the silicone dry for 24 hours before putting the sub in), and losely fill the chambers about 1/2 full with poly-fill (Wal-mart, in the craft section)

If you've got any more questions, feel free to post them.

L8ter,
Josh, aka the tireburner