Here are some pictures of the ‘cat house’ my husband built for our outside ferals. He is a retired electrician so it had to be wired LOL. It is double walled, and floored, with insulation inbetween.
It has two electrical receptacles for heated beds, bowls, etc. And two heat lamps.
It is downright toasty LOL. We don’t use his car much in the winter (I’m in Canada, so road salt is a no no, but it gets pretty cold and snowy). So we have it on his side of the garage.
We have handles on it so we can move it if he needs to use the car. It may give someone an idea if they wanted to build something similar. It didn’t take him long.