You might get some very cheap plywood, cut it to as close to the size of your window as possible, and build an "S" trap in a box to mount in the middle. I've got a storebought light-tight louvre with fan in the middle of my window-blocking plywood, but after being dark for about thirty minutes, I see a faint bit of light comes through even that, so I may have to build my own S-trap box to augment it.

If it's okay with "the management", you could get some double-sided adhesive foam weather stripping, put it around the window, put the plywood on that, and secure it in place with as few brads as possible. It would be better to mount this on the outside of the window, so you can still open and close the window for security/ventilation.

-KwM-