My only thought, and this may be a dumb one, is to secure ropes horizontally every 4 feet or so, threading the ropes through grommets in the bubble wrap or poly, and then pulling them with a rope tied to the center of each width of poly or bubble wrap to spread it along the "ceiling" at night, then open them during the day. I am well aware that there may not be a grommet in your bubble wrap or poly, but I am sure you could find ideas on how to do that online.
The price would be cheap if you used clothes line rope, and they make cheap tensioners so you get get the horizontal ropes taunt.
That is my only thoughts, but it is free and you get what you pay for right?

Best wishes on finding a solution to your problem.