There is a reason not to use halogen for this, I don't remember what it is. I saw it while working on a version of a cookie tin heater. I ended up going with a tiny aquarium heater in a coffee can of water in an insulated bucket. I think the 8 or 15 watt light bulb in a dry tin would have worked too. Without the insulation I would have used a 40 watt or less here in the upper midwest. Where you are, I'd try a 25 watt or less first. If I could rig up a thermal sink around the tin (a second, bigger tin with sand or gravel to set the cookie tin in) (or bricks around even part of it), an even lower watt should work.
You may want a thermal cube. It will switch it on at 35 or so and off at 40 or so.
You don't need the water warm - just not frozen all the way across.