For cat houses outside, take a look at the Feral subforum. There are folks talking about such ones, now and then. You will surely find suitable threads.
Inside? I dont have own suggestions, but as it doesnt need to be isolating against harsh conditions, let your imagination play (depending also on what you do have at hand, or can get easily) and do it. If you are the least technical minded, it shouldnt be too difficult. But good if it somewhat sturdy. As the kittens get a little bigger, they will jump onto the cat house, on the roof.
Having to think it out will be a big part of the fun of doing it, I think!