Im glad you posted this. I was gonna buy a 2' x 4' piece of plywood to make my window thingy(my window is exactly 4 ft wide). But your idea is much better. Having the window completely open, so it looks closed. Genuis.
i cover the whole window inside from bottom sil to the top of the window frame . the only air getting in is through the vents .
& the miniblinds hide the plywood . i keep them 90% closed so ya can't see whats on the other side of them but still lets some air through . not 100% air flow .... but good enough .
as another precaution .... i painted the plywood side facing out .... white . just in case you could see the wood behind the blinds . but like i said ... i check it all the time & i've never seen the white wood so ......