Layer the inside of the outer box with several layers of foam or old carpet padding . This acts as a shock absorbing material between the outer and inner boxes. if the outer box is not triple layered cardboard you can use stiffer styrofoam sheeting sold inexpensively at Home Depot for insulation instead of carpet padding and this will structurally reinforce the outer box . When you are shipping rare irreplaceable pieces DO NOT CUT CUT CORNERS or skimp on packing materials ... If the piece you ship gets damaged or destroyed, not only do you and the new oner loose a good insulator but a disservice is done to the entire hobby ..!