im not sure what the spot light flag is. i never used it myself. but if i was trying to do what you are doing i would make a simple model the shape of your window make it long so it will reach the floor and make the bottom wider than the top. then skin it white or a yellow collor which ever looks better then make it passable and transparent. to make it passable and transparent open the properties pannel and check the boxes.
that is how i would do it.
good luck