200 micron builders film. You might be able to make a frame from tomato stakes or similar material you can buy cheap from the same hardware store. Make the frame a tight fit inside your window frame then wrap the builders film over/around it and staple it on. simply push the blackout frame into your window frame. If it leaks a little light, work at night and expose negatives during daylight hours instead.

This approach has the benefit of being very cheap, easily replaceable and not requiring repairs to the property when you want your rental bond back...

Good luck,

MattC