Generic ported box for two 8 inch subwoofers.
Box will be 31 inches wide, 14 inches deep, 11.5 inches tall. 
Box is 1.65 cubic feet before sub displacment. 
Mounting depth of 9.25 in.
Tuning frequency after typical displacment is 37hz
 
This is a word document with all the information you need to build this subwoofer box, including pictures.