I dont know why they love boxes so much but I bought a 32 Litre Under bed Clear Plastic Box and it has been perfect for my little kitten to play and stay in. These things are just their forte.
I just read your post link. Most of the online sources that I came across seem to agree on a few things. Primarily that this behaviour is instinctive because they can hide (from predators) and feel safe when the walls of a box or a planter come into contact with their bodies.MyCCPay