Some things to keep in mind for your shelter
Maintenance: Shelters should be checked regularly to ensure their optimum quality and function. When deciding what kind of shelter to use, remember that some of the low-cost suggestions will need to be checked and replaced more frequently than some of the more permanent and professionally built shelters.
Doorway: Make sure that the door is only big enough for cats. The door should be 6-8" wide to keep out wildlife and larger predators. The opening should have a flap or an L-shaped entryway to keep cold air from blowing in. If neither option is possible, make sure the door faces away from prevailing winds or faces a wall. Some caregivers prefer shelters that have two doors so cats cannot be cornered.
Protection from the Elements: You can ensure that the cats are protected by making shelters waterproof, windproof (especially in cold climates), and elevated off the ground. Discarded pallets from shipping firms or hardware, farm supplies, or pet stores are a good option for elevation. The space beneath the shelters should be blocked from drafts. Insulation is a good material to use.
Bedding: Straw resists the wet and keeps a shelter warm, and is the best choice for insulation and bedding.
Be sure to use straw—not hay—for feral cat shelters. Do NOT use blankets, carpeting, fake sheepskin, or any material that holds moisture. You can also use hardwood shavings (not cedar or pine), but keep in mind that softwood shavings are not suitable due to possible toxicity. Some caregivers in locations with long, cold winters use Mylar blankets as bedding. Mylar is a product that retains body heat. The generic term for Mylar is Polyester Film or Plastic Sheet. These sheets can be purchased from survival and outdoor stores as thermal safety blankets, or online at websites such as Amazon.com.
Camouflage: Shelters should blend in with their surroundings so that they are not obtrusive to neighbors. Cover them with leaves or other brush or paint them a dark color. Moving shelters into wooded areas away from buildings, parking lots, and other high-traffic areas is also a good idea for cat safety and to avoid neighbor complaints.
Deter Wildlife: Wildlife may decide to make their home in the shelters you provide for the cats. Reducing the shelter door to an opening 6” wide may solve this predicament. Do not use repellants because most of them will repel the cats as well. Some caregivers have resorted to providing additional shelters, accepting that some will be used by wildlife.