I have used boxes as checked luggage many times and have never had a problem; an excellent means of transporting things one-way (like if you are bringing gifts or something). L+W+H must be 62" or less; there is a standard sized box you can buy at Home Depot or Staples or U-Haul that measures 20x20x20, so that's how you could maximize volume subject to the size limits. Boxes are a highly effective means of carrying things, and since a box weighs very little, it helps you stay under the weight maximum (many pieces of luggage weigh several pounds, a box weighs maybe a pound). You will need to sign a waiver, but again, I have never had a problem.