I first built a "play pen" for my chicks that they used from basically 4-8wks. My girls would spend the entire day in it and I started seeing a lot of hawks. When I got their kennel ready I put a tarp over the top and although it was great, it held rain, so I ended up having to cut holes. After the coop was done I moved the kennel up against the coop to create the run, took off the tarp and added deer netting over the top and all the sides, down to the ground. Best stuff ever! Super easy to deal with and cheap 7' tall and 100' long for less than $20.
The kennel as the run is just temporary, we are working on building the final part of the fence to create a bigger run. I'll still be using deer netting over the top of that as well. Chickens will fly and if you do get them out of their pen at certain times I'm sure they will just start taking it upon themselves to get outside whenever they want. My neighbors fence is around 7-8' high and her girls fly out.