George's advice is wise as always. Double boxed custom or factory cartons are fine as long as they're taped down with preferably metal tape onto a pallet with some protective wrap around the box to protect them, and obviously plenty of packing material around the speakers. If you're using a packing/shipping firm, some of them use 'foam in place' machines which can create custom foam mouldings which can be used alongside lots of bubble wrap as George mentioned.
Getting a local tip on a good shipping firm is essential. Forget the online reviews. Good luck!
Getting a local tip on a good shipping firm is essential. Forget the online reviews. Good luck!