I heard geese, peacocks and turkeys (the slim ones, not the fat ones) make good guard birds also. Peacocks have the long display feathers that freak the heck out of predators, but peafowl are quite expensive. In a zoo I watched one of the wandering peacocks who had gotten into the emu pen chase the poor birds around as they were scared of his fanned tail. Turkeys and geese are more aggressive than a guinea fowl but have been added into flocks successfully by raising them with other chicks. The naturally territorial geese fit easily into the role of watchdog and won’t need any training like a dog in how to protect, and at least over here, fertile goose eggs are surprisingly cheap to get, waaay cheaper than a dog.
A single goose would be best as it bonds with the flock of chickens without bonding with other geese, and will attack predators that try and mess with it's flock. Multiple geese may deter predators due to their size, but won't be as bonded to the hens and won't protect them as avidly as a single one would.
Some who keep both turkeys and geese even sometimes have BOTH animals gang up on the threat, so the guineas would also be a good addition as they'll sound an alarm, which'll cause the turkeys and goose/geese to also sound the alarm and then if the threat doesn't leave, they will probably go on the attack. I will feel sorry for the fox if it doesn't heed the warnings from the potential army you may raise from eggs.