There are subwoofer cabinet flat-packs being offered on the diysoundgroup.com website. There are two for 12" drivers, a 1.25 cu.ft. model and a 2.0. Danny recommends 1.5, so you could add some additional bracing to the 2.0---Jon's boxes have a lot more bracing than the flat-packs (they already have one horizontal and one vertical brace, but more is always better than less---just ask your girlfriend). Glue it together, and paint or veneer however you like. They are really nicely made (cut on a CNC machine, with rabbet joints and dado grooves, and a recess for the driver), and bargain priced to boot ($63 and $72.50, plus shipping).
