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).
Keep it Steady
Use sheets of plywood to weigh down lightweight shelters made from plastic, cardboard, Styrofoam, etc.
Cats rely on body heat to stay warm, so keep your shelters small for colonies with just a few cats. For more populated colonies, go with multiple shelters of a larger size.
Warm and Dry
Because it resists moisture, straw is the top choice for insulation and bedding in your feral cat shelters, says Alley Cat Allies. Avoid blankets, which absorb moisture like a sponge.
Yes, cats can get snowed in, so it's important to remove snow from all entrances and exits to their shelters. Shovel regularly to stay ahead of the game.
Skip the Salt
Alley Cat Allies cautions against using salts and chemicals designed to melt snow near your colonies. They can be toxic when licked off paws or ingested from melting puddles, and can hurt a cat's paw pads.
Note that extreme cold weather can increase a cat's energy and nutritional needs. Don't forget extra water to prevent dehydration.
"Wet" Their Appetite
Per Alley Cat Allies, wet food in insulated containers is ideal for cold-weather feeding-because it takes less energy to digest, that's more energy for keeping warm.