I have a new box. It came crammed with goodies for Christmas, but she tells me it is really a shoe box. As you know I am on the large size. Well found. Well covered. Unkind people might even call me fat. I stood in this box for lengthy periods while my human seemed amused. There was nothing funny going on. I was seriously occupied in assessing, measuring,calculating, and eventually I fitted myself into it - to her astonishment.
I admit a bit of me sort of bulges over the side, but my legs and tail and head fit more beautifully. I stay in it for an hour or so in daylight hours and then get out and got for a little constitutional round the coffee table, followed by a snack and a short jaunt into the garden. When I return I settle back to sleep but on my folded blanket which is roughly three times bigger than the box. She offered to show me a photo of a human contortionist in a box but I declined. I am no contortionist. I am a cat. We do this sort of thing.
Yours
Vincent.
PS. This is me in a larger box.
Dear Vincent,

We just do it. Easy peasy.
My friend Bob from Northern Ireland suggests diversifying. He finds plastic laundry binds are an agreeable fit (see photo). If there is some nice smelling clean washing in them, that makes a soft base! Of course he uses traditional boxes too.
But humans don't get the point of boxes at all. I have never seen my human use a box. She just throws them away.
Yours
George