2 Best Options
Option 1: 50 caliber ammo cans, plastic or metal. Store each type of ammunition in a different ammo can. Now store your boxes in a wood or metal chest that is bolted to the floor, and locked with a padlock. If your worried about somebody stealing the ammo, label the chest clothes, or towels, etc. Something people would not want to steal.
Option 2: Put it in a safe.
Both are good options but Option 2 is more simple! Hope this helps!