I can help in design. I've had some visits by hawks to my chickens. However I had installed caging over the top to keep other animals out. This caging provide useful in protecting the chickens from the hawks. I have been looking at building a larger area for my chickens. One option I've found at Lowe's is bird netting. It is lightweight and can be used to cover the top. You may need to put up some posts and use wire to support the netting, but it should deter the magpies. Also take into consideration covering the sides too.