Have you seen the ducts Ted White is starting to sell? It's like an AT flex duct that's cheaper because it's not insulated. You'd run them just like regular flex duct but then fill the cavity with pink stuff.
You have to have a frame of some sort... or do you?
You could do a trampoline rig or tarp clips to hold it in place. The masking would be such that even all the way open, all that would be hidden. You could still take down the masking and unclip the screen for access. If you backlit it you couldn't see what was behind the masking anyway - clips/grommets/whatever are hidden - all you see is the back glow from inside the masked area.
Why build it with that kind of frame? Couldn't you just get the biggest piece of fabric that will fit in the space and mask the whole thing to taste? Curtains do a great job. You could make them out of the black backing or similar AT material. That's the easy way of going CIH. If you want to change the height you have to make masking that's adjustable up/down too. But that's not splitting the atom is it?