While it might be a bit more work, my first reaction would be to go with the igloo/shelter and insulate it for the cats - especially since you are thinking about this WAY before winter gets here. This gives you some time to get everything together, get the cats more accustomed to you being around them, etc.
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My other shelter is an "igloo" type of shelter. O.K., it's really called a "dogloo," but I'm a cat person...what can I say? Anyway, the igloo is not very large, it's insulated (warmer in winter & cooler in summer) and has a small opening. I've placed a sleeping bag inside. That way, the cats have a warm, soft something to snuggle into, plus the sleeping bag itself is insulated for winter weather.
Watch a Cat Build an Igloo in the Snow Video - ABC News
The Harmony Hooded Igloo Cat Bed features a textured plush sleep surface that's comfortably tucked away and ideal for kitties who prefer some privacy when they rest. This hideaway style bed for cats is also machine washable so you can easily keep it fresh for your feline.
MUST SEE VIDEO: Boots the cat builds an igloo - YouTube
End table for you. Bed for your cat. The Igloo Cat Bed gives your cat a hide-a-way that you are happy to display within your home.Basket cat beds are more traditional style and generally take up less space than igloo cat beds. There is a huge range of options when it comes to basket-style cat beds, including , hard plastic baskets, and plush baskets. If you choose a wicker or plastic basket, you should purchase a soft cat sleeping mat or pillow to place in the base of the basket. Consider adding a blanket in colder weather. Baskets provide better airflow and a cooler sleeping environment in hot weather, but should be in a warm spot or near a heat source in cold weather to provide enough warmth for your cat. You can usually launder plush baskets in a and plastic and wicker baskets are easy to wipe clean and keep free of bacteria and parasites.Every cat needs a comfortable, warm place to sleep, rest, and retreat to. Both igloo cat beds and basket cat beds offer a range of benefits for your feline friend. Comparing the two models allows you to quickly and easily assess and determine which is the most suitable for your cat. A snowstorm recently hit Colorado, and a cat owned by Denver 7 Photojournalist Adam Stevens reacted to the weather by digging out an “igloo” through an open window. See the video above for footage of the cat constructing the cozy snow cave.