If running 12VDC, a good charge controller and the battery can dispel this problem. Many of the charge controllers will trip the circuit.
If a GFCI can not be added to an older home or farm, using power strip could be an option. Find one with a very high joule clamp rating to trip the circuit. A Belkin or Tripp-Lite are trusted brands that meet strict UL and TUV standards. I would avoid the cheap no-name or unknown brands since Lazy-L states a good point of the birds getting fried otherwise, or worse, you or a family member touch the watering feeder and get zapped.
Just using a simple power strip means nothing without any protection built into it. In reality, those are just a harder version of an extension cord.
.
OK, in consideration of the GFCI retrofit challenges -- combined with a desire to keep my chickens safe AND being unsuccessful in finding a cookie tin anywhere within an hour's drive, here's my modified plan:
Build a 2 X 4 frame
that you line 100% with concrete backer board -- the kind used as underlayment for ceramic tile. Screw on a full-coverage top and bottom of the same material. I plan to use 1/4" backer board throughout, fastened with 1/4" X 1" hex-head screws (1/2" backer board would be fine, too).
That way, you can use a 120V bulb inside with the comfort of knowing that backer board is
100% fireproof and nonconductive, so nothing can become energized beyond the lamp fixture itself; the juice has nowhere to go inside of the concrete backer board and your chickens will be 100% safe.
The backer board may take just a bit longer to warm up, but once it is, it should keep the waterer free of ice. The point is that with a concrete backer board lining, top, and bottom, a cookie tin is completely unnecessary -- just mount the light fixture inside of the backer board-lined wood frame. Also, elaborate electrical safeguards are unnecessary.
Nevertheless, a Thermo-cube would be a good idea to prevent unnecessary overheating.