When I was taking care of all the outdoor stray kitties, I bought dog houses with the door off to the side so that the kitties had a solid wall to snuggle in behind. I put a heating pad on low underneath blankets, then draped a blanket over the top of the dog house to keep the wind out of the opening and to further insulate it. The dog house had legs so that it wasn't sitting directly on the cold ground or cement. You can buy them at Pet Supplies Plus and they're pretty solid; they cost $80 for the medium size. I houses an awful lot of kitties that way because the families snuggled together. When I bought my new house, I gave all but one dog house to the vet tech who took several of my strays at that time but I still have one left, just in case a stray kitty crosses my path. You might want to consider this also. It sure eased my mind.

Blessings,
Mary