I would just use plain top soil (no fertilizers or perlite) for substrate in an outdoor enclosure. To make it predator proof, dig fencing into the ground a foot or so all the way around, or bury a bottom a foot underground. Make the sides of the pen tall and put a top on it. Make sure at the torts level he or she cannot see out. Putting a lock on the door/lid is also not a bad idea to keep out people and smarter animals (like raccoons). Any plants you put in just make sure they are non toxic.