This is a great idea. You would be surprised how high chickens can go. My neighbor used an old book shelf and made a big nest box out of it. It was mounted 4 ft high. Surprisingly the birds got up into it and layed their eggs.What if either the door to get in the coop was higher, with a ladder or steps a chicken could get up but not a dog... or the nest boxes are up higher, again, so a chicken can fly up but not the dogs. You could have a ladder or perch or something halfway so the chickens even do it in two stages.