Gnu, a friend gave me an old, heavily constructed stereo component box, with a door on the front. I cut a cat size hole in one side of the door. It's about 20X20 inches, 18 high, and has two inch sticks on the bottom so it is raised a bit off the deck boards. I lined it inside with blue foam sheet insulation, sides and bottom, and soft foam on top (just because I had scraps around). I put an old thick fake sheepskin construction worker type vest in for a bed - it goes up sides and back and covers floor.

Then we started having all these rainy windy days, and water was going inside, so I covered it with old raincoat, shored it up with slab wood, and made eaves for it (more slabwood, the inventive non-carpenter solution) to stick out about five inches to help that problem. There's about four to six inches of slabwood all around it now, further insulating it.

It ain't a pretty deck ornament, but I have a dry and reasonably warm outdoor cat. If we get oldfashioned really cold weather, it may not be enough.