Shelter From the ColdA shed, garage, or barn would make ideal shelter for feral cats and outdoor owned cats. It is almost unanimous that straw is the best bedding for keeping cats warm, since hay or blankets trap moisture. The hay can become moldy, and the blankets will be cold and damp. If you use a garage, be extremely careful of antifreeze leaks. Antifreeze with ethylene glycol is extremely toxic, and also sweet-tasting and attractive to cats.
Lacking a suitable building, use your inventiveness for a structural shelter. How about under a porch or a high crawl space under your house? Since cold and frost flows down, overhead cover is more usually important than four walls, for shelter from cold. Lacking any of the above, you'll need to be inventive in designing a small shelter for outdoor cats. Here's one example:
Foam-Insulated Cardboard Packing Boxes
Appliances such as washers, dryers, and refrigerators usually come in well-insulated cardboard boxes. If you don't have large ones like that, you can make do by taping smaller boxes together to approximate the larger sizes. You'll need two boxes, one approximately three to four inches larger than the other. Lay the larger box with its open side on top. Fit foam packing material to the bottom - either the solid foam or foam pellets. Securely tape the smaller box's top closed, then set the smaller box inside the larger box, bottom side down. Fill in all spaces between the boxes with foam packing material, using a solid piece in the side facing you. (This will be the side with the door opening.) Using a carpenter's pencil or a wide permanent marker, draw an opening about one foot square, starting about two inches from the ground. Using a sharp knife cut the door opening through all layers. While holding the foam securely in place, tape well around the entire opening. Line the back part of the house with straw for bedding, and put food and water bowls in the front on either sides of the door.
Before I rescued her, my Jenny was a vagrant, semi-feral young cat, who came to me looking for attention and food. During the cold, rainy California Winter, she could not come indoors yet, as my husband had terminal cancer, and her presence would have been too disruptive to our other cats. I provided her with shelter in the form of a medium dog-sized Igloo Dog House on the deck of our former home. It was large enough for an outdoor electric heating pad, which I put under a sleeping blanket, and food and water dishes. Read more suggestions from readers on Keeping Feral Cats Warm During Cold Weather.