How many ferals do you have? How about styrofoam and wood chips or straw? I been looking around for you.
When Annie was feral, I put one of those styrofoam picnic cooler things on my porch.
Hope these help! The wind chill today is supposed to be between -20 to -30!
http://www.all-creatures.org/ak/feral-shelter.html
http://www.alleycat.org/warm.html
http://www.for-the-love-of-cats.com/...ldWeather.html