We found these tips on catsofel.com
Feral cat caretakers, here are some quick tips to help you and your colonies get through the cold winter months:
Ask your community to donate storage bins (like Rubbermaid). Line the bins with Styrofoam, cut out a doorway and voila! Instant shelter.
Build it Better
Got handy staffers or volunteers? Neighborhood Cats offers detailed instructions onbuilding the ultimate feral cat shelter, seen at right.
In a Pinch, Raid the Trash
A cardboard shelter is better than no shelter. To keep it from getting wet, elevate off the ground, line with newspapers and cover the lid with plastic (a garbage bag will do).
That’s Heavy, Man
Use sheets of plywood to weigh down lightweight shelters made from plastic, cardboard, Styrofoam, etc.