There wasn't really any plan. I just sealed up the box and set to with a pair of scissors and some hot glue. I made it up as I went along and reinforced where it seemed weak. It wasn't helped by the cats who started playing with it as it was being built. (Actually there's a cat across my lap and arm as I type!)
As you can see, it is quite popular and I also had a play with some structural engineering ideas too.