This is definitely true. Po is a little different in that he will sit in/on anything new, it doesn't even have to resemble a box.
It could be the smallest box that he'll squeeze his big butt into, but it's also everything else. If I move the laundry basket to a different room, that's where he will be. A piece of paper falls onto the floor? Expect Po to be sitting on top of it in 2 minutes flat. Yesterday I put a sheet on the floor and he decided he had to sit there because it was something new! Anything and everything. Here's where he is now. He couldn't get inside so he sits on top. He just can't help himself.