Ahh, I see the disconnect. I didn't use Uhaul. The company I used is called "Upack". They are owned by ABF Freight. The boxes are a bit bigger and made of some type of metal, I would assume steel. We fit out queen size set no problem. The video in the link below shows them packing one with stuff including a king size mattress. I hear ya though. Moving is a pain in the ass. The convenience of them transporting everything is great though. Hopefully my comment helps somebody in the future.
ReloCubes are 6’ x 7’ x 8’, with 308 cubic feet inside, which is usually enough space to move everything from one large room.
www.upack.com