Skip the cardboard, it will not help you if the speakers are really abused as they often are when shipped ground.
Make some crates from 1x4 rough cut lumber, add 2-3" on all measurements, line crate with cardboard inside wood and then foam or bubble wrap, wrap speakers in plastic or cloth and put them in. This will be much more secure if you are shipping them UPS or the like.
Make some crates from 1x4 rough cut lumber, add 2-3" on all measurements, line crate with cardboard inside wood and then foam or bubble wrap, wrap speakers in plastic or cloth and put them in. This will be much more secure if you are shipping them UPS or the like.