Buy a wolf
Seriously though, the problem you'll have in trying to deter dogs, is not driving your own dog mental in the process. Foxes are very clever animals, so the chances are, all these fangle dangled ultrasonic devices, flashy things and water jets are probably all going to work in the short term, until such a time as the fox figures out they're no real harm.
A better suggestion might be look at how the foxes are getting in, and prevent that, rather than waste time and money on these so called deterrents which in the long run probably won't work.
Giving power to politicians is like giving whiskey and car keys to a teenage boy - P. J. O'Rourke (thanks Jessie)
It's amazing that people can believe everything is predestined but they still look both ways when crossing the road - Stephen Hawking
1 Wife, 3 children, 1 Staffie Bitch (RIP Marley), 1 Chi-Chi, 1 Tuxedo Cat, 1 part Maine Coon cat, male bearded dragon, Horsefield Tortoise, 2 White Silkies, 1 Frizzle Pekin, 1 CLB, 1 Appenzeller Spitzhauben Cockerel, 1 blue laced Wyandotte, 3 Appenzeller x Wynadotte pullets, 1 Call drake, 3 khaki Campbell ducks, 4 (2 male 2 female?) Aylesbury x Campbells, a breeding colony of Dubia cockroaches.
And a lot of Ibuprofen.