90% ofman made earth building is the positioning ( the "perfect site" is no good if the foxes don't run near by).
checking an earth with a terrier has not seemed to make any difference ( no need for the dog to enter if he doesn't think an one is home)
shooting/dispatching a fox in the earth has ruined them ( iput this down to the fact that concrete pipes hold the scent/ pherhormones etc better than soil/sand of a natural earth).
the building material seems not tobe that important either, i have used plastic pipes,drainage pipes ( peforated) ,concrete pipes,clay pipes,slabs,bricks nad