@aart gave you a good suggestion. I have a hardware cloth "apron" around the perimeter of both my coop and my run. It works because critters who are intent on getting in don't have the sense to start digging 18 inches away from where they want to be and work their way over....they start as close to the fence as they can and then they hit that hardware cloth barrier. For that you can even use chicken wire rather than the more expensive hardware cloth, but that was a corner I didn't want to cut.
Mine goes out about 2 feet, and is anchored to the ground with landscape fabric staples, then runs up the fencing of the run as well. We were going to put flat rock over it, but the grass grows up through it so well you can't even see it. We can mow right up to the sides of the run, and that keeps hiding places for smaller critters down.
Does doing it that way work? Yep, it does. When our chickens first went out to the run, our English Setter Molly got way too excited. She hustled up and down that run and then decided to dig. She broke and bloodied a toenail and came away limping. She shot the chickens a dirty look over her shoulder and laid the total blame on them - and she never bothered the chickens again, even if they were free ranging. She associated the "owie" with the chickens. Stoopid dog!