Thanks to everybody for your responses, I definitely appreciate your help. Wayne thanks for the interesting math problem but I appear to be a bit out of my league here(at least with the port calulations).I have calculated that the larger box less the volume of the 2x2 bracing will leave me with an additional 1000 cubic inches of space in the cabinent. I will just assume that the large port hole will be close to what is required and see how it sounds. It may actually be pretty close.
I will get back to you soon as I am just about finished with the one box and will try it out before building the second. I have learned by doing that I can sometimes avoid mistakes or make things easier on the second one.