I use some version of the pet heating pads sold on Amazon for my outdoor boxes. The last one was made by Marunda. They are all around $25 and work great in the bottom of a medium pet carrier with a towel over the top to help insulate. I take the doors off of them.

If you are making cat beds without heating pads, I’d say to use straw for the bedding and one of those self heating mats with it. Even my indoor cats love those things.