I have a very small flock of 6 - 8 hens and a very small coop. I looked for water heaters to keep water from freezing in winter and found that they were expensive and too large. I decided to try and build something that would be cheaper and fit better and this is what I came up with.
I picked up an electric grill at a garage sale.
I turned it upside down and took the guts out of it.
I then cut the end off a small extension cord and threaded a couple of pieces of flashing and a screen door spring on to it. Then I replaced the end.
I then threaded the cord through the side of the base, putting one piece of flashing on the inside and the other on the outside.
I used a couple of small bolts to hold the flashing in place. This holds the cord and keeps mice out.
I picked up a plugin light socket at the hardware store and a small incandescent bulb. 25 watts is plenty.
I then turned the base upside down onto a fire proof material, in this case a suitable sized metal pan but any fire proof material would work.
A small plastic water font fits perfectly on top. I have used this heater for several years now and it keeps the water from freezing in the coldest weather. No more wading through knee-deep snow to de-ice water fonts two or three times a day. Cost was less than $10 and worth every penny.