Generic ported box for 2 15 inch subwoofer.
Box will be 38 inches wide, 28 inches deep, 19.5 inches tall. 
Box is 8.4 cubic feet before sub displacment. 
Mounting depth of 17 in.
Tuning frequency after typical displacment is 33hz
 
This is a word document with all the information you need to build this subwoofer box, including pictures.