My goofy hens have dust-bathed close to the framework of their run, leaving gaps underneath. We never installed an apron, as we got the chickens long before we ever heard of BYC. I suppose we'll install one retroactively but meanwhile the coop is still secure even though the run is not. *Always...
Although I'm a fine one be even the least bit critical, we leave the man door to our coop open fron daylight to nearly dark, anything could just walk right in. We've had a possum in there a couple of times but it seemed more interested in the chicken feed than the chickens. He won't be coming...
Four sizes is interesting to me too, I had no idea. What my XH called river rock - he was in construction, bridges, highways and the like - was bigger than a softball and smaller than a bowling ball. It was also used in rip rap. What @fowltemptress showed looks to me like what he would have...