You might want to consider black plastic shower curtains, the thicker kind that you can buy a Target, Wal-Mart, ect. They work great at blocking off a window. Whatever you use, your biggest challenge will be making sure you create a good seal so that light doesn't "leak in" around your barrier. If you want something removeable I would imagine you could probably attach one to a frame and use some sort of flexible padding (car door rubber surrounds?) to fill the gap.
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