Thanks for your feedback, jiggsaw.

I am planning to do a scrog, since I'll be flowering using CFLs and won't have much light penetration. So, I'll definitely be doing lots of training of the plants to spread them out properly over the screen. Right now my plan is two plants for this grow space, though I would ideally like to have a separate small veg chamber for a mother (or for clones and flower the "mother" instead).

Now that I have pondered it for a couple days, I think I'm going to build a veg chamber on top of the cross-beams, and the flowering chamber below. That way I'm actually using the beams to separate one from the other and bear the weight of the veg chamber. 40" will be tough for a scrog, especially when you add in the height of the soil and container, but I should be able to manage I think. Since the beams will be the ceiling and floor of each chamber, I will not be restrained by the 24" depth.

Anyone else have input?