i was looking at your numbers and something seems off.....
you have a total length of 31" (on the back of the pic)
you have 13.125 + .13.125 + .75 + .75 + .75 which would be 28.5 on the front
31 - 28.5 = 2.5.
that would leave 2.5/2 = 1.25 for the total port width of each side.
but if you measure the length of your port wall 16 + .75 = 16.75
and the length of the center port wall you get 19.75 - 16.75 = 3" opening
you sould have a 6.75" (including the divider) port opening on the front of the box....not 2.5+.75 - 3.25
if the divider was not in between the ports you would be a lot closer to what you need to be....3 open to each chamber and 6 at the front of the box....i think it was made a little backwards....unless i'm missing something....