'Predator proof' is a big deal, and expensive, especially for a larger area. 'Safer' is an easier concept to work with.
So sorry for your losses! It sounds like a fox, who will be back.
A large safe run includes a dig-proof foundation, either a skirt, or concrete down about two feet. Walls of hardware cloth, with 2"x4" woven wire over the lower four feet, all well secured, not only stapled. A roof able to handle snow load, because it needs to be solid, or if hardware cloth, snow will pile up on it anyway. Doors that can't be torn or opened easily. Solid.
For a large area and ranging, electric fencing works well for ground predators, and sheltered areas helps for raptors.
Mary