I built one of those cookie tin heaters, 3 yrs ago. Probably the best $5 (Less than that actually) I ever spent on the birds. I use a 40W bulb in mine. Check the link.
Jack
https://www.backyardchickens.com/a/cookie-tin-heater-for-water
Jack
https://www.backyardchickens.com/a/cookie-tin-heater-for-water