Thats a good plan! That may work.We have two ultrasonic repellers in the closet. We pulled them because it bothers the dogs, cats, chickens, etc. Maybe that far away it wouldn't bother anyone, but I do want the dogs to go down there and they'd set it off.
Once after a fresh snow I saw where a dog fox was marking the same place. Fox have become so habituated to human scent/activity that it does little to deter them. Snaring, trapping, shooting would be effective in eliminating the fox.