I thought my spare room was small at 13' x 14' where I am eventually planning on a projector and M80's. I have the same problem where I would have to mount the screen over a window.
For the window problem, my plan is to build a shallow box that will mount over the window. I'm not sure how to fasten it yet, but I could always use hinges on one side and a latch of some sort on the other. Then build a similar box for the other side of the wall so it would be symmetrical. Then I would just need to make sure it would block out all of the light. I would also have easy access to the window if the need should arise.
I have one window on the right side of the wall where the screen would mount and another window on the wall to the right of the screen.
