I'd take the tile off the top of the block, and insulate the inside of the block
The difference is your bowl is only heating the water and itself, while the bulb is heating the block, the tile, the bowl AND the water
Also, how many WATTS is the dog bowl?
How much water os on the block?
That determines the BTU's available
You'd also get better results using HALF a block, insulated inside.
All your heat production is going into the base itself
The dog bowl is 60 watts, and built far more efficient than this cinder block heater.
I have only metal on the top of the block now. The cement should heat up, and radiate heat, in theory at least.
Insulating the hole where the bulb sits,hmmmm, keeping the heat in, but allowing it to radiate up to the top metal cover and the bowl..
1.5 gallon of water is in the bowl over the block, but the heat transfer on the larger surface area would be pretty high, so I do not see this as available method to keep water from freezing in my climate. maybe where it is not far below freezing, but not in sub-zero weather. The heated dog bowls do handle the deep cold pretty good...