Thank you fionasmom and jeanw. For now, I've decided to start with:
K&H Pet Products Otdoor Unheated Kitty House, Olive, and
Pet Magasin Thermal Self-Heated Cat Bed
from Chewy. It's not purrfect but it's better than nothing. I just don't think that I would keep up with a microwaveable pad...
I know nothing about cats, but there's a stray that comes to my door. I'd like to buy a little shelter to put out that it can go into during the cold winter. Can someone suggest which one to buy? Brand? Model? Seller? URL? Any info appreciated.
Also, how do I keep rats and raccoons and such...