You can easily make a welded wire 'apron' facing out all around the run to protect from large digging predators. To keep out foxes, the fence itself must not be some cheesy chicken wire, as they can bend and rip it. If the fence is chicken wire, laying down a hardware cloth or welded wire apron would be useless. To help keep mice at bay, a raised coop (even just five or six inches) is very helpful, and so is putting down bated traps where the chickens cant get to them. During the day, chickens will eat mice, but during the night mice will nibble on the tail feathers of chickens for protein and eat their feed.