In this system a 3D printed latch and pawl are used to clamp a plate to the windowframe to seal it up. It allows you to lead the duct of a portable air conditioner out of a window, while keeping the window sealed to prevent warm outside air from entering the room.

You'll almost certainly need to adjust the pawl to match the thickness of your sheet of wood/plastic, and adjust how far away from the plate it grips. Fusion360 file is provided for that purpose. If you need help adjusting it, let me know.