I use a high pitched rabbit squealer.
But if you want consistent fox action, find someone who raises chickens!! Set up as close as they'll let you to the coop at last light and call aggresively and get ready to shoot!!!
I used to have a good number of chickens. Not a "commercial" number mind you, but just a little hobby thing. I could NOT keep them alive this time of year!! Between grey fox and weasels chickens weren't safe at any time of day or night.