For as long as I can remember, Lily's "spot" has always been an old Priority Mail box on my desk:
It was just the right size for her, easy to replace when it got grungy, and I could move it around the house with me to keep her out of my hair while I was crafting.
Lily is a small cat - just under 9 pounds. Her sister Tonks, on the other hand, is twice that at 16 pounds, and had no chance of fitting in Lily's box.
This is a clear case of "square hole, round giant blobby cat."
Tonks has worshipped Lily since she was a kitten, though, and so dutifully follows Lily - and her box - where ever they go.
Then one day I witnessed the impossible: TONKS GOT IN LILY'S BOX.