I too have used cardboard in the back yard as the basis for a shelter. Bad idea. Although the cardboard used to be wood, it’s a bit too far removed from the tree to really be useful, at least in my case. You see, I thought the snow would remain cold enough against the cardboard so it wouldn’t get wet. I knew wet cardboard would lose it’s integrity, but forgot about my own body heat. Well, it just turned out to be quite a mess. A limp plastic sheet turned rigid with rope and sticks is much better than rigid cardboard turned limp by moisture. As has been said – observe the birds and animals. They like to stay warm and dry, too.