If you're not-all-that-handy, and if you don't want to spend $100 per cat house, there are lots of easy inexpensive shelter options to explore. A Styrofoam cooler filled with straw, with an entry hole cut into the side, will usually be perfectly acceptable during a cold snap. Elevate the cooler from the ground to help it maintain heat. Many medical offices receive medications shipped in Styrofoam coolers and will be glad to share them with you for free.
There are also endless ways to turn a large plastic storage container into a cat shelter. Just Google "storage tub cat shelter" and you'll find an option that will work for you. But, just so you don't have to look too hard, here are some easy designs we like: