This isn’t a major project but very useful. The shelf is a 1×10. I used my sawhorses and Porter Cable 7-1/4″ circular saw to cut it to length. I offered to stain it but my wife liked the natural color.  I didn’t want the shelf to be permanent nor do excessive damage to the drywall so I used #12 2-1/2″ finishing nails around the perimeter for the board to sit on.

Simple 1×10 shelf