Living in a rural area, as I do, I have constructed several wooden houses for them to sleep in at night. Generally made out of old planks from wooden fences and other scrap wood. They are made to keep the cold air out, and mounted on the roof or high up in the work shop with carpeting or blankets inside. I have a few ferals and some that do not like to spend the nights inside, so they sleep in these boxes.

But yours is a good idea Randi, for those who do not have access to scrap wood, electric saws and dills, and dry-wall screws. (the main building materials and tools), especially about the styrofoam, as it is one of the best insulators against cold available. And the "price is right".