Don’t know if this is the best place for this post, but feel free to move to somewhere better.
As a child I had a beloved outdoor cat. In many ways he was my closest buddy, he had the temerment and loyalty of a dog, but hunted mice ferociously.
One winter, the nights were dropping to below -20F, and although I corralled him into the garage, it was un-heated and maybe 5 degrees warmer than the fridgid outside.
We set up a little cat retreat in the garage. This consisted of an old doghouse, really a crate used to transport a dog via airport. The crate was cheap to be certain. I swear the plywood was 1/8 inch thick. I can hardly believe it survived the trip, let alone the next 15 years, but it did, but it offered no thermal protection to speak of.
But inside we placed a laundry basket lined with an old sleeping bag. We placed an old blanket draped over the crate that helped to give the crate a door, and while my cat liked his soft new home, it was still plenty cold.
I wanted to add heat, thought about adding a lamp inside but didn’t like the fire potential. I remember from school that water has a crazy high specific heat capacity so I decided to fill 3 1-gallon water jugs with hot water and place them at the back. I stuck them in and waited 2 hours. After 2 hours I stuck my head in under the blanket and was greeted by a blast of warm, probably 90 degree air. My cat was purring loudly.
Before we went to bed, we replaced the water, and boiled water to fill one jug, placed it in between the other two hot jugs, tucked the cat in for the night and went to bed.
The temperature dropped to around-25F that night and the garage only got colder. In the morning I stuck my head back in and my cat was as happy as could be! Nice and warm for hours! The water jugs definitely cooled off but we’re still lukewarm with the center one being plenty warm still. On cold nights, this became the standard way to keep my cat warm on bitter cold nights.