Don’t overthink it. I’ve got a bedsheet strung on one of those closet poles that holds itself up by pressure. It’s over the doorway from our hallway to the kitchen. The living room and hallway are comfortable (I did close off the bedroom and bathroom doors), while the kitchen is still approximately 99238 degrees, exactly as it is outside.
Now, is it the best insulator ever? Of course not. But, at least with our layout, we’re not losing much coolth past the sheet. Not enough to drive my heat addled mind into total meltdown thinking of better, more expensive solutions, anyhow. 