A Box Sling
Any suspended object will tend toward the lowest energy state.  In the diagram above, the center of gravity will drop lower if the two halves of the sling slide toward each other.  To prevent this tendency, separate lines, shown in red, are tied to keep the two halves of the girth hitch apart so that the center of gravity remains well within the footprint of the rope supports.

Notice that the center of gravity, shown in gray, is directly below the pick point for the sling.  If you try to hoist a load with the center of gravity not directly beneath your pick point, the load will rotate until the C.G. is directly below the pick point (its lowest energy state).

You can
bend the ends of the blue line together to complete the sling.  In critical cases, protect the rope from the edges of the box.  Remember that not all boxes can take the loads induced from hoisting.

Related Page:  Figure 8 Barrel Sling, Cat's Paw