I acquired an unfinished double-hung window for a playscale dollhouse from Ebay. It came with a removable plastic pane. I took out the pane and spray painted the frame white.
I then measured two pieces of fabric the size of the pane from my backdrops collection. I ironed each piece of fabric and used Mod Podge to adhere it to a slightly stiff sheet of watercolor paper.
When I get some adhesive magnetic strips, I will put some on the back of the window frame. I can then pin the pieces of fabric between the frame and my magnetic backdrop so that the scene appears to be either day or night.