They do sell microwave heat discs that can be added to bedding or to stray. You heat them in the microwave and then add to the space. When I used them years ago, they stayed warm 4-5 hours. You do need to reheat every day.
When I cared for my feral boys outside, I had a hound heater in an insulated cat house my DH built. I also used outdoor approved heat pads. We had electrical outlets outside, but we used many extension cords. The heat pads were the absolute favorite. The hound heater worked great. It was silent, but it would mean he would need to go into an insulated cat space.
https://www.amazon.com/dp/B0014LJKUA/?tag=thecatsite
https://www.amazon.com/gp/product/B07PFYLYW8/?tag=thecatsite
https://www.amazon.com/gp/product/B000ICGJZK/?tag=thecatsite
When I cared for my feral boys outside, I had a hound heater in an insulated cat house my DH built. I also used outdoor approved heat pads. We had electrical outlets outside, but we used many extension cords. The heat pads were the absolute favorite. The hound heater worked great. It was silent, but it would mean he would need to go into an insulated cat space.
https://www.amazon.com/dp/B0014LJKUA/?tag=thecatsite
https://www.amazon.com/gp/product/B07PFYLYW8/?tag=thecatsite
https://www.amazon.com/gp/product/B000ICGJZK/?tag=thecatsite