Just an idea to throw out: I saw a stray cat on the other side of my property (13 acres) before Christmas. I will not trap in this bitterly cold weather, but I did haul a plastic dog house over there (other side of hill); the containers the others have mentioned will work just as well. I used thick, rigid styrofoam to insulate the bottom and then made a "mattress" out of some unused bubble wrap I had stashed around. I happened to have a bubble-wrap "bag" and stuffed more bubble wrap, making sure it was flat, into the "bag." The result is something padded and comfy to lie on as well as being a great insulator - all that trapped air! I added an old "cuddler" bed (the round kind, with sides) and a self-heating pad. I'm not able to get over there often; when I checked a few days later, the house & bed appeared unused, but at least it's there if the cat - who may not have survived the last single-digit nights - is still here. Anyway, remember the bubble wrap - cheap, easy, and a good insulator.