The recommended vent size of 4" dia by 12.25" long will tune the 2 cu ft (net volume) box to about 33hz, and be about 3dB down at 29hz.
You may have to use a 4" PVC elbow to fit in a shallow box. Measure the distance down the middle of the elbow (it's usually about 8"~9") and add a short segment to make up the total length. Dry fit it before cementing in place. Do not stuff, line one side, back, and bottom (only) with 1" fiberglass (or similar).
