I use my greenhouse (6x8 Harbor Freight GH) in the winter for my chickens. My waterer is a 2 gallon galvanized unit......the double wall anti siphon one. I have it sitting on top of a terracotta flowerpot base. This is all sitting on top of a single cinder block with 60w bulb inside of the cinder block. I ran the cord and extension cord inside of 2" pvc so no cord is visible to the chickens.
I couldn't find a 40w bulb so I have used the 60w all season. We have had some 14 degree nights that got down around 25 in the GH and the water stays drinkable and not edible....frozen!
I know there are probably better ways, but for me, this has worked really well and I don't have to worry about frozen water.
hope this helps
I once used an exposed bulb similar to yours.
As an FYI, some water dripped on the exposed bulb while replacing the container, the hot bulb shattered and I had to deal with tiny splinters of glass.
The terracotta flowerpot base is a great idea.