Think outside the box
I've found this one strange for a long time so I was pleased to finally tackle it.
I do most, if not all, of my thinking outside boxes so I'm not sure why this phrase gets such a regular outing. I think in houses, offices and cars which can be described at being a bit boxy but aren't actual boxes. The only way I could get to the bottom of this would be to think in a box.
I needed a box. A box to think in. A box big enough that I could actually get in.
I popped over to my parents' house. They still have the box their television came in. My young nieces sometimes play in it. It's a boat or more often a spaceship to them. To me it was just a box.
My parents have been used to odd behaviour down the years but I don't think they were expecting me to get into a large cardboard box and sit quietly. They first enquired into what I was doing ("thinking") and then to the state of my health ("I'm alright, leave me alone for a minute").
They were relieved when I finally got out of the box and carried on helping them with the crossword and enjoying a cup of tea as if nothing had happened.
And thinking in a box? Much the same as the thinking outside of a box. Only more cramped and darker.
Conclusion: Inside a box, outside a box. It all seems to be the same. Do your thinking wherever you see fit. But see fit to do some thinking.