As it is fairly flat, but long and wide, I was going to see about a large cardboard folded in half lengthwise. What are the dimensions, if I may ask? I'm looking at a box from an IKEA cabinet and it is about 2' x 6' x 4-5" high. If the lip would fit, I think it would be a good size box I could save.
I'm amazed at what goes for shipping sometimes. I've even seen large, odd shaped items shipped with just a FedEx label attached to the item without packaging even.