Why not beef up your tractor so the fox can't get in. Maybe some fencing pieces on the ground around the exterior that you set your run on and something stronger than chicken wire on the run. I have foxes here and if you look for where a fox could get in you can prevent that from happening. Besides fox are very good at keeping rodent populations down. All predators will eats chickens, it easier to learn how to keep predators out than to try and catch or kill all of them.