Yes, weird as it may seem, cats
I think a few plain old boxes will do, but if you really want to make something:
Well, for all 3 litters of kittens I've had- wait, are your cats still kittens? Well either way this will work -I made a little 'house' for them out of cardboard boxes. It had a box tipped on its side for the momma to feed the kittens, and then a 'hallway' which led to a bigger box, laid down normally, which we called the 'playroom'. And then from there, there was another very long 'hallway' which led to the 'bedroom', where they usually slept. All of it had pillows and sheets throughout. ANYWAYS, you could make something like that for your cats....Even if they are grown cats.
Not sure if they'd like it as much as a plain old box though. I'm not sure. But you could try it!