Many years ago when I was renting an apartment and didn't want to leave any damage to the kitchen windows when the time came to move out, I just covered the windows with black plastic sheets (crawl space stuff). I wrapped it around the outside of the window frame, doubled it over and stuck thumbtacks through it. No light came in through the edges.
If I had wanted to be able to remove the covering to open the window, I probably would have made a wood frame around the outside of the window frame with either black rubber tubing or self-adhesive weatherstripping stuck on the wood frame at the wall interface, then attached the black plastic to the outside of the wood frame with thumbtacks. To support the wood frame in place, I'd have screwed eye-screws into the wood frame to hang on L-hooks placed into the wall above the window. Pressing the wood frame against the wall to compress the light seal would not be difficult.
Here, too, storing this covering when not in use might be problematic, but I'd be inclined to hang it on some pegboard placed on the wall.