Cats don't need a lot of space, just a space that is large enough for them to stand and move about and stay safe from the harshest outdoor elements. When the weather is at its coldest, cats will rely upon each other for warmth and will create their own tight spaces within their shelter. With that in mind, you can use whatever space and materials available to you to create a small haven out of the elements.
Homemade shelters can be crafted out of nearly anything: from a sturdy cardboard box to an anchored plastic storage bin or a few pieces of scrap wood that have been arranged as a buffer against rain, snow and wind. The shelter should be large enough to accommodate several cats comfortably, but not to wide or tall.
When creating the entry to the shelter, keep in mind that the opening should only be large enough to allow the cat to enter, so that the shelter interior remains dry. If space allows, you can create an awning or plastic "curtain" to shield the entry. You can also place the shelter close to a wall, with the entrance facing the wall.
If you are able to put a little more time and material into the shelter, try adding insulation to the interior roof and walls of the structure and caulk seams to create a draft free space. Bedding material can be added, straw is excellent option as it retains heat and does not hold onto excess moisture.
Place the shelter in an safe concealed spot where the cats feel secure and hidden from predators (including humans) and can watch their surroundings.
See a picture slide show of how to build a cat shelter here.
Access to Food and Water
Cats will required extra calories and fat during the cold weather months in order to maintain their energy requirements. A dry kitten formula is an excellent source of extra calories and balanced nutrition. Canned foods are also a good source of nutrition, but may freeze in colder temperatures.
Food and water stations should be protected from the the elements and placed near the shelter, so cats do not have to be exposed to harsh conditions when they need to eat and drink. Heated water bowls are affordable and will help prevent freezing. Feeding cats at the same time each day will allow them to expect and rely upon a schedule.