I've seen people use something like this on top of their fence:
Even if you do that, I would still suggest keeping them in something smaller first. I'm working on the same kind of problem. I'll either be buying a couple 'chicken tractors' or a shed with a catio attached.
The place I'm buying was already fully fenced with smaller mesh fencing for chickens, so I'll add a top that leans over to keep them in, but, still will likely keep them in the smaller enclosures for a month first so they know where home is if they do manage to get out.