You've had some good suggestions so far. I've used structural foam from a store like Home Depot or Lowes. Get the good stuff, not the white, brittle stuff. Once you find decent boxes, use at least 1.5-3 inches of structural foam on all sides, top and bottom. Get a couple of different thicknesses, if necessary, to add up to the width you need. Use a drywall T for marking and as a straight edge for cutting the foam with a utility knife. Make sure the wrapped speakers fit tightly in the foamed boxes so there is ZERO movement. This will eliminate breakdown of the foam and possible abrasions. Clean the speaker with a damp cloth, then dry, before wrapping the speaker with stretch plastic kitchen wrap. Stretch the plastic wrap tightly around all sides, top and bottom of the speakers. The plastic wrap will eliminate any abrasions.
I've shipped a few different speakers and audio components this way and the packaging I created was WAY better than most factory packaging I've seen.
I've shipped a few different speakers and audio components this way and the packaging I created was WAY better than most factory packaging I've seen.