You can put a standard padlock on the door of the trailer while it’s at your location — you’ll just need to remove it before the driver picks it up. You can also place a lock on the bulkhead, which stays there until you remove it at destination. The bulkhead lock should have a shank size of 2 inches minimum length and ¼ inch or smaller in diameter. (The lock you would use for a trailer hitch will typically work.)