I am not using a cookie tin, but have a 40 watt round bulb in a concrete block with one of the plastic gravity feed containers. The water container is secured by twine with an elastic band which holds it in place. The climate here in Tennessee is usually mild. So far this year the coldest it has been is around 17F. Tomorrow night is predicted to be closer to 10F. The system has worked fine so far. I also have a separate neighboring coop with PVC pipe and nipples. I have a heated cable on the pipe which goes through a wall and is fed by a bucket, which is actually in the coop with the other heater and younger chickens. The heat cable, which is controlled by an automatic thermostat, is fifteen feet long and ends up around the bucket. So far, that has worked also. The pictures were taken while installing and they don't show the heat cable.
I once used an "open" system very similar to yours.
When filling the water, I had the misfortune of a small water spill hitting the bulb.
Needless to say, small splintered glass everywhere. Just an FYI...