I decided to make this as a photo instructable as it's really pretty simple. We got two new cats recently and as they're to be kept indoors we figured an activity centre would keep them happy. Then we saw the prices. Instead, I picked up a carpet inner cardboard roll from a carpet shop, some old boxes that were lying around, some string and a handful of washers. Then I actually spent money(!) on a bag of new sticks for my hot glue gun and set to work!
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.