I currently put a box fan in a window as an exhaust fan when I need to ventilate a room in my house. But when I turn the fan off, I need to remove the fan and close the window. I'd like to have a system with louvers that automatically close when the fan shuts off so I don't need to worry about hot air or bugs coming in. But I don't want a unit that requires any alterations to the window frame. Is there such a thing?