i built a very big run several years ago (perimeter of approx. 150 metres.which our girls live in permanently apart from the odd weekend out in the veg patch when theres no damage to be done. yet to lose one to the fox and there certainly about. key points 7 foot high, double layer of mesh for first 3 feet, 2 foot skirt which I tent peg down all the way round , grass soon binds it tight to ground. and make sure the gate is not a weak spot.