Generic ported box for four 8 inch subwoofers.
Box will be 48 inches wide, 17 inches deep, 12.5 inches tall. 
Box is 3.4 cubic feet before sub displacment. 
Mounting depth of 9.5 in.
Tuning frequency after typical displacment is 36hz
 
This is a word document with all the information you need to build this subwoofer box, including pictures.