Best to put them in a box that fits the antlers. Fill it up with news paper nice and tight and you should be good to go, You could wrap the horn tips to help not pucturing the box. The UPS store has a good assortment of sizes.
I was in the corrugated box manufacturing business for 41 years, corrugated is very strong when sized right and the correct strength is used.
I still have access to a packaging engineer to make my St Judes shippers if I need something special, but most can be found at the UPS store or the post office along with some good interior packing to be sure the package is tight. .