Inexpensive Projection Screen

We have a used overhead projector (the kind teachers use to project transparencies onto a screen). In our homeschool classroom, we don't have a blank wall space to project onto, so we mounted an inexpensive pull-down window shade (from Home Depot) onto the top of the wall near the ceiling. It functions just as a projection screen would. It retracts back up when not in use and is out of the way. Thought I'd pass this little idea along...  Here's a pic: