How do your customers handle inserts in your boxes? Seems that many products need some really custom insert to keep from rattling around.
There's another material called dunnage i.e. fill that is placed such as air packs, bubble wrap, paper, cardboard, mushroom (yes a product made from mushrooms!), etc. are used to keep the product tight.
Another method is to use a smaller box which cuts down on both shipping costs as well as is a more secure way of securing products.