Now I’m wondering if there is a battery powered heater for outdoor pet houses. Do you know of any such thing?

I saw a patio heater but I think it ran on kerosene.

any.

I laughed to myself once thinking of putting little solar panels on the slanted roof.

This company makes electric pads for poultry, outdoor dogs, cats, bunnies, etc. I have about 5 of them that I use for cats. Never tried these microwave ones.

https://cozywinters.com/shop/kh-3111.html

They're so lucky to have a caring humom to look after them!

Get a used dog house. People give them away on Craig's List and in newspaper advertising. Generally free for pick-up. If not, buy one used. Clean it up. Paint the exterior; carpet the interior. Wind proof it. Then install an electric dog house heater of some kind. They sell them at Amazon and Home Depot and Lowe's and Tractor Supply and at other hardware stores. You may even find a "used" dog house which already has one installed in it.

https://www.amazon.com/Dog-House-Heater/s?k=Dog+House+Heater

I've been browsing dog houses. My brother is an avid yard sales guy so he's on the look out for one.

So be careful not to overheat the area or they might choose to chance the cold.