How to completely blackout (at night only,) a 45" square window

Lowes sells black paper accordion shades. I know they sound cheesy but once you hang them up, you can always fold them up out of the way. They do work though. I have them in 2 bedrooms and they totally block out any light at night. So dark that you can't see your hand if you hold it directly in front of your face! And during the day it cuts the bright light by 75%. I am one of those people that have to sleep in a dark room and these shades do the trick.

I would use a blackout drape. They can be bought at most big box stores that carry linens.

Blackout Blind or Curtain, available ready made or made to order.

block out shades

Since you want something temporary, I would put up a tension rod and hang something from it that can be flipped up during the day. Or are you looking for something that can be permanently affixed directly onto the glass?

They do have black out curtains and shades or, as the previous person said, use a tension rode and hang curtains from that.