I do allot of shipping in/out. The rule of the house is box comes in. cats have first choice. I will some times retape the box then cut a hole in one or two ends. It's fun to see a 15 lb. cat try to sit in a 6 inch square box or a kitten in a 40 x 36 in.
When the box gets crushed or worn out there is always a new one available.
Having different sizes is great fun for all.
Right now there is a small box in front of the stove Demetri, Lucky, and Roz take turns in it.