I have a 39"x39" tent and the 6" ac infinity as an exhaust fan. Before last night I was running the tent with the vents open and the duct ports partially open. Just last night I installed a portion of 6" duct in each of my lower duct ports and curved them around some to prevent light leaks and closed 2 of my 3 vents.
Now, on 10 speed, it sucks in a little, but not much. I'll be scrogging, so my scrog frame will keep the tent expanded once installed.
If you're not scrogging you can install a bracing system. On my old 4'x4' tent I used aluminum angle to keep it open, I've seen others use wood.