I use two pieces of cardboard covered in that shelf liner. (I had all these cardboard boxes and shelf liner left over from our move) Then as I need I adjust the two pieces to allow more or less ventilation. Plastic wrap would work fine, as well as plexiglass. You want something that is nonporous that will trap the humidity in, and not suck it out. Use what you have on hand. Maybe even plastic wrap a piece of cardboard if you have to. Whatever works.