Making places for feral cats better and more secure is easier you might think of. If cats live near your house, on the streets or in your courtyard, it needs just a little effort to do something for their better life. For protection from rain and bad weather you could use a box with a cardbox on it. If you put the cardbox into a garbage bag and cut an opening in it as an entrance, you have a simple cat house. Additionally, you could put straw into the cardbox. O.k. this is not a beauty, but it is cheap and helps the cats during bad weather and low temperature. It is even better than sleeping under cars during the winter and gives them the feeling of privacy. Necessarily you should provide a litterbox, which will be cleaned up regularly, too. Doing this helps you to avoid trouble about the cats with your neighbours.
To keep away parasites like fleas from the cat house and from cats itself, you could use diatomaceous earth (diatomite or kieselgur). It is so fine, that fleas and other external parasites suffocate. But please use it only outdoors and never in the house. Otherwise you won’t get rid of the powder.