Go to your local pizza restaurant and see if they will sell you some boxes. Be aware that these may not be strong enough to mail in. Most of the pizza boxes are made of fairly lightweight cardboard (like cereal boxes), not corregated or carton cardboard like mailing boxes. Also the post office has size restrictions, 20x20x6 may be over their limit, so I would ask at your post office before buying the boxes. (UPS and FedEx also have size restrictions, so if this is the way you are shipping, call them first to ask.) I would go with mailing tubes personally. These can be purchased at the post office as well as some of the independent shipping stores and are much sturdier than pizza boxes. Good luck!
Anita