Backstory: I live in an area with a lot of strays/abandoned cats. For the last 6-7 years I've made friends with a bunch of them (roughly 25 so far.) I've been leaving food out, and have been bringing them in to get fixed as the little bit of money I get allows. (around 15 so far, been focusing on the females although a few boys have been done too.) Found homes for a number of them, but a few new ones keep showing up every year.

I live in Canada, our winters regularly drop below -30c (-22f) and much colder. In the spring it's not uncommon to find a dead one in the neighborhood that had froze during the winter. I have an insulated box outside that has saved a few from freezing to death, but it's not heated and isn't very big. I have very little extra money but I want to do what I can for the little furballs. (I've been wanting to upgrade my 15yr old pc for years now, but this is more important to me.)

What I want to do is take an old broken apartment size chest freezer that I have and turn it into a cat house with a pet heating pad to keep it at least a bit warmer for any cat that needs it. I was going to cut a cat flap into the side for them and make a few floors or levels inside. I think it'll help, but I'm a bit concerned about how the air quality will be in there. Will the cat flap let enough air through, or should I vent it as well? Is there anything else I should be doing as well? If anyone has any suggestions or improvements on my idea, I'd love to hear them.