I checked it out, and see it as a good product. It is UL and CSA certified. I would certainly have it plugged into a GFIC outlet.
I don't think there is a ledge large enough in width for your chickens to decide to walk on.
You don't include much details which heater you have. I would protect it from weather regardless. You can make something out of pallets, and cover with clear plastic. This way chickens can get into enclosure, and drink without the snow on ground.
WISHING YOU...