I believe (and I could be wrong) that weasels aren't much for digging, however they will follow rodents through their burrows, so the aim should be excluding rats and other burrowing rodents.
A 12" apron may or may not be wide enough to exclude a rat from getting in in 1 night, as I've seen a rat tunnel under my apron about that long.
@rosemarythyme That's helpful, thank you. Well this answer explains at least in part the strength of a mobile system. If a rat does decide to tunnel in, the problem is solved, in a sense, when the tractor is moved to a new location. Chicken poops are left behind, leaving interest outside of the coop, not just inside. And with an open bottom coop like above, they're not going to be able to nest in it.
@aart &
@MadGardener, yes I don't relish the idea of including an apron, as it does seem a lot more work to move the tractor. I get that if it's not flat ground, it may become obvious to a predator that there's an ingress, still, it would slow them down at least, maybe enough for us to hear the chickens fussing. As far as pinning it up, yes, I was picturing the one I saw someone here mention and link to, I'm forgetting the creator's name, but it featured a smartly designed and inexpensive hinged apron, held up against the side wiring with dangling lawn staples, the same that can be put to use in pinning the apron down. Great system.
I've done some preliminary research into preventing rats (
here), and it seems that an apron is one recommended deterrent. Rats can dig down by up to a meter, it says (!), but will be foiled if they are trying to dig at the location of the hardware cloth.
Editing my post, as I missed the last post from
@MadGardener. Waffle stomping, lol! Yeah, enclosing the bottom seems a pain, though I can understand while people do it. You make great points, I've been thinking about all these things a lot. Definitely most tractors seem on the small side, especially this one. I've been planning to get electronetting. I think combining that with the above tractor would be a great system, and I would skip the apron in that case. That would be way easier. Only thing is I'd need a net, which seems cumbersome to move, or some other deterrent for the numerous hawks around here. I'm beginning to lean towards a secured coop tractor with a larger run. I'm planning on six chickens.