The mistake people often make when tying milk crates to their racks is looping a single rope or bungee under the rack and up to both sides. That allows the crate to shift easily and dump over the side if the bike is tilted enough.

Instead, use separate pieces of rope or short bungees to secure each side of the milk box independently to the carrier deck or legs. Done that way, there's no potential for it to shift position.

Another trick is to use nuts, bolts and washers to make 4 pins that stick out from the bottom about 1/4". Position them so they fit just inside (or outside) the rails so the rack cannot move when it's placed on the deck. Then all you need it tie it down, with the pins securing it from sliding.