If the box involves some "mechanical" or "physical" new elements (e.g. a new hinge or way of opening), you may wish to consider patenting those elements, too. As to design elements, you can and should protect them under both copyright (which applies to the specific graphic elements and/or text) and trade mark (which applies to unique design or graphic features which identify you and your business as the source). In short, you have several options, and the best thing to do is to consult with an intellectual property (IP) attorney to evaluate which will help you the best.