My latest, and seemingly effective solution is to wrap the speaker in two layers of 1" beadboard, put 4-ply cardboard corners on all eight corners, and put all of this in a box. I ship the plinth separate from the main cabinet because the cross-section of the plinth is larger than the cabinet and was taking all of the force.

My experience is that all of the damage is done in the last 100 feet of the journey. The drivers can't lift the boxes, so rather than slide them gently off of the truck deck, they just push them off. The box drops 3 feet onto one corner. The cardboard corner blocks were the solution. They spread the force over the beadboard and keep the corner of the speaker from penetrating the box. I have tried wood crates, but the drivers hate them -- hard on the hands -- and make a speaceal effort to damage the shipment. Wood boxes are brittle enough to shatter unless they are prohibitively heavy. Cardboard boxes are more likely to stay together.

I have a problem with lining the box with plywood. At my weights, the plywood might well cut through the box. What are your shipping weights?

Bob