Outdoor cat houses can be as simple as a converted foam cooler or an elaborate wood construction. Outdoor shelters are often used to protect feral cats.
To make a foam cooler shelter, use duct tape to affix heavy-duty garbage bags around the outside of a cooler. Cut a hole into the front for the cats to enter.
A more durable option is adapting an 18-gallon plastic tub into a cat house. Cut a 6-inch square opening in the front of the tub. Place a paving brick in the bottom, and line the tub with egg cartons for insulation. Install a layer of Reflectix foil over the egg cartons for heat. Insert straw into the bin, and secure the top with duct tape.
Another option for building a cat shelter is nestling two storage boxes. Place a foam board in the bottom of the larger box, place the smaller box inside, and cut a hole for the cat. Reinforce the hole with tubing, and place straw between the boxes. Affix the top with duct tape.
AlleyCat.org offers elaborate plans for building shelters out of plywood as well as recycled items, such as garden sheds and camper toppers. The site also features several kits for purchase.
Building wooden structures are more durable but more involved. KittyCatHouses.com has 17-inch to 20-inch cat house plans with platforms and lofts.