I have a small colony of cats on my block, and this instructable shows how I make the winter shelters I provide for my TNR'd cats.
Step 1: Gather Your Equipment and Supplies
Styrofoam fish/meat shipping boxes
Silicone caulk and/or construction adhesive like Liquid Nails
straw or other bedding
paint & appropriate applicator
shrink wrap or garbage bags and duct tape
bricks or scrap 2x4's
Step 2: Prepare Your Fish Boxes
These boxes come in many different sizes, but we need to make sure that they will fit a cat (or preferably two), and that there is room for a door opening and for the cats to walk in, turn around, and walk out. 12 to 18 inches high by 2 to 3 feet long is a good size. If your boxes are tall enough and have a good lid, then just use your caulk gun and adhesive to secure the lid on to the box and move on to the next step.
If you are like me, then your lids are missing or destroyed, or your boxes are too short. that's not a problem. Grab that old crappy steak knife you keep in the utility drawer in the kitchen and using two boxes of the same size, cut off the lip on the top of the box that is meant to secure the lid (see picture). You are probably tempted to use a box knife like I was. Don't do it...the blade length and handle configuration makes it hard to get a straight flat cut here. Besides, it's nice to have a purpose for those "free bonus gifts" you got in your last purchase of the new kitchen gadget as seen on TV.
Step 3: Secure the Top, Fill in Holes
This is pretty simple.... just grab your caulk gun loaded with adhesive and run a bead all along the top edge. If you're working with a tall enough box, just plop the lid on and squish it down, or if you're working with a pair of shorter boxes, invert your matching box on top and do your best to align all the edges. Weight down the top to ensure a good contact at the joint and wait for the amount of time recommended for the adhesive you're using.
After your joint is secure, you may want to go back around it on the outside with some silicone caulk. You can also use the caulk to fill in any holes or cracks in the foam. Basically, just do what you can to seal out drafts and leaks.
Step 4: Make a Door
On one end of your box, use your ruler and straight edge to mark out a square of about 5-1/2 inches. Make sure that it is an inch or two away from the ground. This is important to keep the bedding in and water out. Cut this out with your trusty junk drawer steak knife.
Step 5: Finishing the Outside
Step 6: Installation
Now for the bedding... As humans, our instinct is to offer pillows and blankets like we sleep with. Keep in mind that cats don't have the thumbs required for pulling up the covers. Most of the expert materials I have read recommended using straw, and I concur. My ferals spent a couple of days turning their noses up at my fish boxes, but the minute I put the straw in, I swear they thought a new "W" had just gone up in the neighborhood. Just go to a nursery and buy a bale of straw (not hay), and stuff some in. Many craft stores and grocery stores may also offer bales of straw near Halloween for decorating.
Just add a cat and you're done!
Step 7: Conclusion and More Information
For more information on how you can help feral cats in your community, check out the ASPCA website, or look up a TNR program in your area. In New York, a good resource is Neighborhood Cats.