Foxes right now have babies in their dens. Being kept in the run isn't really bad for the birds. If you really want them to have some extra space, use metal U stakes and run a 4 foot high fence of chicken wire. You could even include the run if you wanted. Get a livestock electric fence charger and run a wire along the top of the fence as well as along the bottom. You could even make three or 4 strands and run that a foot out from the chicken wire on a second row of stakes. Most foxes will not be out in the daytime but they might be strapped for food if they are feeding pups and be desperate.
Putting up an electric fence is easy. If you made a separate barrier of 4 or 5 strands, first one 4 inches off the ground, next three 6 inches or so spaced. Fox and other ground predators won't cross it. Put pieces of bacon over the wire in select spots (do not have the bacon make a connection between wires) and after a sample, mr fox will not be back.