FedEx ground. They beat UPS hands down. I use them every day.

If you want to get fancy, build a simple plywood box. (sounds goofy, but we bought 4 model a doors on ebay, built 2 boxes and sent them to him to repack)

But we get heads in every day at work from Mercruiser and Volvo, and they come in heavy cardboard boxes. Go to your local engine rebuilder or marina, and see if they have any left over. We just toss them when we are done.