What gap size for fencing would keep out a fox?
We have no raccoons or snakes, it's just foxes and rats to worry about. We have had no problem with rats previously, or at least none a terrier didn't solve, and I don't think it would be feasible to modify the run such that rats couldn't get in...
I have an old, shamefully neglected little chicken coop which was absolutely fine (save the need of a lick of paint etc). Except I'd never checked the external nestboxes. The lid of that was built out of a different material and covered with a black sheet of roofing material. It got wet...