Most likely you know most of this, but just in case:
1. Cats are survivors. Cats can stay warm on their own. It’s really when they get wet that their fur doesn’t work as well. A simple all Coroplast shelter with a bit of bedding material is very inexpensive way to protect your cats and keep them dry.
2. It’s best to use straw for bedding. I believe pine shavings are good as well. Here in Houston, we don’t have easy access straw, but I’ve found that Tractor Supply Co. does carry both straw bales and pine shavings. Pine shavings are only $5.00 per bag and they go a long way. You can fill 10 – 15 shelters with one bag.
3. Keep your shelter off the ground. If you build a Corokitty shelter the feet are meant for this purpose. If you create a Rubbermaid shelter you can use 2x4s to elevate it from the cold ground. Keeping the shelter off the ground helps them stay warmer and ensure rain won’t get in due to flooding.
4. Cats don’t typically sleep more than two in a shelter so a small shelter is best as it does a better job keeping the warmth near their body. This is very similar to the way blankets and jackets work. The Corokitty shelters are built more for one cat, but two could get in their if they wanted to. This small space, and arched roof, allow for a warmer shelter even if you do not use insulation foam in the interior. Bedding is important – so make sure and use the straw or pine shavings.
5. Use catnip. Catnip in the shelter will get them in where they’ll realize it’s nice and cozy.
No matter what kind of shelter you build make sure to use some nice warm bedding. Sheet insulation (you can get from Lowes or Home Depot) is great for reflecting the cats body heat with an R value or 3. The Coroplast adds a bit more R value and some straw or pine shavings will help to keep the inside of the shelter dry.