Hi, i have made the run smiliar size as urs, i have put mesh on top aswell so its got mesh all around, fox can only dig under the run but not tried digging so far but im going to digg around it and put some chicken wire in burry it. I have put the bricks underneath the run, well on the edges so the run is standing on the bricks rather than on the ground so it doesn't get as wet. U can search for my posts i have put some pictures up.
It's more safer to lock up ur chooks properly just incase the fox does get in the run somehow, but if u think its 100% fox proof then u can leave it open.
Hope it helps :-) sorry for the essay lol
Mo