Well, yes, that's the point -- I want to build the biggest box I can in whatever dimension should be biggest. I can either build all the way to the roof, but squeeze it between two ceiling beams that are 24" apart, or I can make it short enough to fit underneath the beams (which are 40" high), but then as wide and deep as I want (up to about 60", when it runs into the A/C duct work).

As I said, I think I'm going to build the flowering chamber under the 40" high beams, and put the clone/mother chamber on top. That way, only it will be squeezed between the beams, and the flowering chamber can spread out low as a sog or scrog.