I have a small flock and they never wander far from the run and coop.
You can establish invisible boundaries without too much work...Or maybe I just got lucky.
When I got my first order of chicks, I kept them in the house for the first 3 months, handled them often. I built up my coop and run in that 3 months time. I put them out in the coop first, locked them in there for about 3 weeks, then I opened up the coop into the run and let them roam around between coop and run for another month before letting them out to free range.
We live in the city and have a fenced in back yard, my birds have never once left that yard, they do go outside the fence in the back to where we have an old swingset with bird feeders and they eat the feed that falls on the ground. But they never leave our property.
You may try this same thing when you build your enclosure, Once you establish home base for them, they wont wander far from it.
I also had a hawk attack a few weeks ago, since then, there free ranging time has been cut down and only under supervision. The other poster is right, the hawk will return for your birds, I have seen the same one lurking around here waiting for his next opportunity. He wasn't lucky enough to get my hen, just took half her feathers before we chased him off. But he isn't going anywhere unless I dispatch him.
Good luck to you, let us know how it goes.