The woofer should be centered horizontally, offset slightly below center vertically and the port should be below that, centered in the space that remains. Put a pair of cross-braces, one pair spanning side to side, and the other pair spanning front to back. The top pair should be placed 1/3rd the way from the top, the other 1/3rd the way from the bottom. Each brace should fit snug, providing a little bit of force on the panel.

Put R11 or R13 insulation inside on three sides, either left or right, the bottom and the face side opposite the woofer cutout. Also hang a sheet across each brace, spanning the cross-section of the cabinet. The insulation isn't required for a subwoofer or bass bin, provided midrange is not presented to the woofer. But this cabinet does run up into the midrange, and the insulation damps internal standing wave modes. It attenuates midrange standing waves but bass passes right through.