I also use the postal service triangle boxes. I figure (hope) the P.O. workers might show it some respect, since it has their logo on it. It's good to reinforce them a little. Lightweight sheetrock corners, glued in the corners, give longitudinal stiffness.
I think it helps to fill all the space inside the box. I spray foam the bow in place on one end, stuff it full of air cells or bubble wrap, and foam the other end.