Hi there.
My best friend's husband was raised on a chicken ranch for 18 years. He and his wife have 3/4 of an acre just a few yards from a creek/urban greenspace area. I don't know about foxes, but they have opossums, skunks and racoons for sure.
When they began keeping chickens he built the coop on a concrete pad. They only let their chickens free range under supervision. They built a very large run. He used a mower-sized mini tractor with a digging attachment to make a trench 2 feet deep and buried 1/2" hardware cloth that far down. It extends up to become the run fence which is held to a frame made of 2 X 4s braced to each other with those metal joiner things (haha. I'm no carpenter!). The fence is 7' high and is topped on the outside, so climbers will touch it before getting to the inside, with a hot electric wire. It runs insulated up one of the posts and is held out from the fence proper about 6 inches by arms that tilt down and outward from the top fence rail. Under the ground for a couple of feet outside the doorway and for a foot inside it he buried many lengths of barbed wire running parallel to the fence and about 1 foot longer than the doorway on each side. They're buried about 3 inches below the surface of the soil. Instead of rat-proofing the coop he stores the food in metal containers. I don't know what, if anything, he does about rat-proofing the feeders. He also has wires stretched across the run, from fencetop to fencetop and hangs CDs and mylar strips to keep hawks away.
I read about a man who would get the old, sweet-tasting antifreeze and use a syringe to inject it into hotdogs. He would squirt in as much as they could hold, then he would pour it all over them like a sauce. He would leave it out at night for the predators to snack on. He said he killed a lot this way, but I don't know if you can get that kind of antifreeze anymore.
They haven't had much trouble with predators so far. I hope you have better luck with your chicken. You've gone through enough expense and heartache already.
