How about a large (and in charge) lightweight pvc tubing or similar frame with chicken wire around it - have your husband help you lift it to a new area of your property every day or whenever you think the hens have exhausted
the area.
I'm making something like it for my roommates chickens she is going to get because of stray cats - like a chicken tractor but the size of some people's backyards haha!!
The fox could still eventually get at the chickens but wouldn't be able to for a bit until he dug or whatever and that few precious seconds or even minutes when the chickens see him and sound the alarm might allow you or your silly dog to scare it off and the chickens will still get lots of exercise and food but will be safer!
Also, I may have a tip for dog training. When i lived elsewhere a few years ago, I would see a stray cat in the backyard I would make a sharp "SSSSSS" sound to let them know get out or I'll chase you out and I would stand and act tense like a dog on guard (you know how they have their chests out, frozen eye contact and tails up?).
You can make whatever sound you like but the dogs seem to respond to more exciting sound than commands like "go get it" so could use a hiss or something haha.
Anyways after a few times of that sound, my dog (who likes kitties and wouldn't touch them even if he did chase them out of the yard) would LEAP UP and stand on guard and look where I'm looking with his tail quivering, ready to go if I let him which I don't because I do the sound for raccoons and stuff too lol but it could be easily transferred into stimulating them into hunt mode! And he's "just" a big old adopted mutt named Leroy who's half boxer and half shar pei - haha!
To this day, even if I haven't made the sound in months if I make it he leaps up and looks and stays statue still unless I release him, if you made that sound every time your chickens made distress sounds and went over there all tough lol like you expect to find a predator, your dog might love that and link chicken distress calls to RUNNING to them for the hunt!
I hope everything works out for you!