Get 2 boxes big enough for the heads with some extra space, 4 garbage bags, and a can of minimal expanding spray foam insulation. put a garbage bag in the bottom of the first box, opened up. Spray foam to fill the bottom of the bag, fold the bag in and set the head on the bag. Take the second bag and put it unto the box over the head, pushing it down around the sides. Fill the bag about 1/2 full of foam, wad the bag up and close and tape the box shut before the foam expands too much. Repeat with the second box/head. The foam will expand to fill the voids, keep it secure in the box, and cushion any hits the box takes. When they open the box on the other end, they can just pull the top foam piece out and grab the head.