I have had great success with forming a grid of Mason Twine across the top of my run. My run is 6' tall and 28'x35' so it took a LOT of twine, but works well. I heard commotion from the hens one morning and went out to see a hawk sitting on the top of the fence with no way to get in to the hens....he was frustrated, I was angry, I won! The grid is run about every 8"-12" over the entire run.
hope this helps!!
hope this helps!!