Where I live, Free Range Chickens = Free Chicken Dinner. I keep my chickens in a fenced in chicken run during the daytime and lock them up in a Fort Knox coop at night.
I don't worry about a fox digging underneath my chicken run fence, because I lock the girls up at night in a predator proof coop. I have never seen a fox snooping around my coop or run during the daytime.
If you are concerned that a fox might attempt to dig underneath the chicken run fence, you could maybe dig out a trench about 18 inches deep and put wire fencing down that far. I like the concept of just laying down a predator proof apron of about 24 inches of wire on top of the ground around the chicken run fence. That apron sounds like a lot less work, and if it works as well, that is what I would do first.
I imagine a free chicken dinner is pretty tempting for a fox and family. I would expect them to come back for more as long as you have chickens running around. I hope you get a good chicken run built soon and stop the losses.

You might want to contact your local DNR and see if they have a program to capture a nuisance fox or at least give you some advice on trapping one that is killing your chickens. The guys at my local DNR are pretty helpful on stuff like that, but I suppose it all depends on the people working there. Good luck.