This is a sub I built for a friend from work.  It's a 15" TC Sounds TC-3000 subwoofer in a 100L sealed enclosure with Parts Express's HPSA1000 plate amp.  The box is built from 3/4" MDF and covered in 1/4" oak veneer.  There's 4" of R-11 fiberglass insulation lining the sides and bottom of the cabinet.  The front baffle is 1.75" thick and the overall box is heavily braced with window-type bracing and cross bracing using 3/4" MDF.  The target Qtc for this design is around 0.65.