box building

I'm trying to build a sealed sub enclosure for 4 Kicker L5 15's with dividers.I have my trunk measurements and I've calculated for the internal volume.I want to know what do I have to do to get the correct  internal air space with the dividers.My measurements are as follows: width=32 height=14 depth=31.Each speaker require 1.5 cubic feet of air space.Do I need to add more to the external volume ?

if those are your external measurements and you are using 3/4 MDF just subtract  1.5 inches from each measurement and calculate the volume again.  then find the volume of each of your braces and subtract them from your internat box volume and that will be your total net volume.