Okay, so this is where we left off last night.

When I checked on it last night, however, I found that the aluminum foil did not prevent water from dripping down. After looking around for something that might work, I decided to commandeer one of the containers we have been using for the cat kibble. A heavy bottomed frying pan that the handle had broken off, long ago.
Some time during the night, however, it got knocked over. We could see the light glowing through the drainage holes in the terracotta pot through the bathroom window. When I went out to do their food and water in the morning, I found a small ice patch where the water had spilled.
I am guessing that, when it got to hardly any water left, the cats knocked it over while trying to reach it.
I decided to go with the concrete block.

I am hoping the weight of the block will make up for it resting on the edges of the light fixture. It seemed high, though, so I moved it closer to the kneeler, and put the pot and tin upside down near it, so the cats could sit on them, if the wished.

Which promptly got ignored. :-D
At least with the block, though, he can put his paws on the corners to stabilize himself.
When we checked it again, hours later, it had been knocked off again, but it was also completely dry. There was no new ice. Again, it seems that they knocked it over only when the water was almost gone.
So I’ve put it back, and filled it with more water than I had before.
As for the heat; it is clearly working, in that the water is kept from freezing, but neither the pot nor the block seemed to warm up at all on the sides. There is no radiant heat coming off. The heat of the bulb is enough to keep the water from freezing directly above, but not enough to fight off the cold on the sides. I had hoped it would provide at least a little bit of warmth that way, but it I’d probably have to go up to a 100 watt bulb to accomplish that, at which point I think it would be too hot at the base of the pan. That cats, at least, know how to use their own body heat to keep them warm! :-)
The Re-Farmer