Use an old screen door window, some white spraypaint

And you get a cheap whiteboard that would work better than my current solution of shower board.  OK time to keep and eye out for folks tossing out big windows/screen doors.  Oh and sales on white paint.  Nice thing here is the windows are usually already framed .  The one thing I would do different is either frame the window in, or make holes in the frame itself to hold it to the wall.

http://www.instructables.com/id/very-cheap-DIY-white-board/