As an ex pheasant keeper,
Set the fence bottom 18 inches below ground level, have an over hang of at least 12 inches on the outside of the pen, also foxes dislike the smell of stale human urine, so hang jars of the aforementioned around the fence too
if this fails to deter Reynard:
Halogen lamps connected to PIR sensors, a flask of strong black coffee, and a 12 bore shotgun, loaded with BB shot cartridges, these have the effect of stopping Reynard, dead, in his tracks
hth
Ubd