I think if you take HappyMats approach and tweak it a bit you can easily get the result you desire if nothing so far has worked.
If you take a plane that is bigger than the box, with the same window opening in it, and put it infront of the window. turn on shadows for the light so that this plane will only let light through the hole. Then in the render settings turn it to a non renderable item and the light will act as though it is there but it will not render.

Does that work for you?

-Alexander