1/2" plywood and 2x4's is what I would recomend.
I wish there was an easy way to scetch on the internet....
Take your measurements, and figure out your cut list. You need to make square frames with the 2x4's to attach the plywood to, and then nail the pieces together.
(If you can give me the dimensions of the pieces, I can make a list for you.)
Spray the heads with an rust proofing oil (Rust Prevent 500 is what I have always used on the job - my work is using something new, that is supposedly cheaper now. I can get the name if you wish.), wrap in VCI (Volitile Corrosion Inhibitor) paper, and at least cover in VCI plastic (regular plastic will do) to prevent worst case scenerio of sitting out in the rain damage to them. Brace in with 2x4 pieces.
Of course, this may be too much for the "home user," but this is how I would do it if I had to ship them. Rough guess: If the heads weigh 50 pounds a piece (and there is two of them) I would say the total package weight will be around 150lbs. If the set weighs 50lbs, then the total package weight would be around 80 or 90lbs.
That is a great deal of shipping cost right there......
Are you sure you want to ship them?