take a 50 cal ammo can, cut a hole with a dremel from the inside toward the latch, thru the latch. Then insert a the U-shaped piece from a gate lock hasp combo and put a lock on it. As long as it doesnt rattle - Ive gotten away with transporting ammo this way.
Another easier method is to use a plastic pistol case, remove the foam and put it in your checked baggage (with your clothes) - just tell them there's ammo in the suitcase. This is totally legal as long as its not in the same case as your rifle.