For what it's worth… Jim Salk ships speakers by FedEx ground. He packs speakers in two cardboard cartons with a layer of foam between boxes.
First, the speakers are covered by a protective sleeve (thin flexible foam, I don't know its name) and put in a corrugated cardboard box. This carton then goes into another larger sturdier cardboard box. The space between the outer box and the inner box is filled with 1" thick styrofoam "boards". This seems to withstand FedEx handling.
You might contact Jim Salk or Dennis Murphy and ask what packing materials/methods they like, and see if you can find them locally.