I made a couple of ammo holders out of an old 2 piece arrow quiver made by hoyt. Both the broadhead end and the end that holds the arrow shafts will work for just about any common sized round. I used the lower shaft holder for .243. The one for holding the broadheads can be made to hold any size you want. You just custom fit the foam insert.
Either piece can be mounted to a tree or stand (as mine was) with a little thought as they are already designed to be mounted to a bow. I'm sure there other brands that will work just as well. Probably could find them pretty cheap to darn near free, if you look around a bit. Internet, rummage sales, buddies, you know.