I provide shelters to three feral colonies. Large Styrofoam boxes, about three feet long, have a doorway cut in one end, straw to a depth of 5--6 inches lines the bottom. Three small drainage holes in the bottom in case rain gets in. The boxes are surrounded by a contractor bag, and I top the boxes with large sheets of plexiglass for protection. One sheet is good for to boxes, as I have them side by side.
A local city wide animal group sells the Styrofoam boxes all ready with doorway, straw, drainage holes, and contractor bag for $20, but one of my neighbors gives me a different Styrofoam box that he orders Omaha steaks in. Look around, I'm sure you can find a free source, although the work of cutting a door, etc takes about an hour per box