I was talking to my sister the other day. She was telling me that she left a box out and her cat was in it minutes later sleeping. Her cat loves boxes. Does your cat, too?
I was thinking about this when I saw this blog post about a product called "Catty Stacks". Catty Stacks are boxes with holes in them that your cat can climb into. They are stackable and can hold up to 20 lbs.
It looks like you can stack them in a variety of ways. The boxes have holes on every side. I don't know how well they balance or if you need something to make them stick together more securely. In the photo, it looks like they have tunnels you can attach in between each box. Kind of looks like a HabiTrail for cats!
I think my sister's cat would have a ball in this. The whole problem is the dog would probably jump on it to get the cat out.