I'm building a tractor just for them to run around in the yard with, since I'm afraid to let them out free because of the potentially-testy neighbors. There can be no possibility of them getting into another yard (in addition to dogs in the other yards being a problem). We have a dog who's pretty laid-back; I'm not too worried about him with the birds. There are neighborhood cats, however. We live near Portland, OR, so not many other daytime predators. They'll be in their coop in the garage at night and won't often be unsupervised even in their tractor. However - I really want it to be at the very least cat-proof, and should some neighbor dog somehow get into our yard (really not much danger of this), I'd really like it to stand up to heavier assaults. So, I think two things must be important: tip-proofness, and strength against battering by large mammals. As it stands now, for temporarily housing 4 hens, the tractor is 6x3x3, and I haven't decided what construction for the roof. Seems like a sloped roof would be stronger than a flat one as well as less predator-accessible (harder to stand on). Suggestions?