Hello, I'm moving across country and have a question about what I am able to use as a checked bag. I have one duffle bag but it isnt enough to pack all my clothing. Would I be able to use a cardboard box taped heavly to take the rest of my clothing with me. The box is 12inches tall, 15 inches wide, 21 inches long and under 50 lbs. Would I be able to use this box as my second check bag or must I get a second bag?