Panda film is pretty thick, you can get it in different thickness but the thickest Ive seen is 6mil which is pretty darn thick. Thick enough to even sew on a zipper so that wouldn't be an issue.

Here's what you could do, have it open while light is on-this way you can have a fan in there for circulation. When it comes time for the light to turn off you can close it and no light will go through that stuff. Only thing is you want to be able to get fresh air in and old air out somehow, so maybe a small vent system.