> The flat side faces the road

It would probably work a little better if there was some durable insulation on the "road" side, enduring more of the heat transferred to the pan.

But, as JCL says, nothing works as well as a real block heater, with the heating element directly in internal contact with the coolant, mounted in a freeze plug opening.

I would note that in very cold climates, such as below 0 degrees F, a block heater is used not only to ensure the engine starts, but to reduce the wear and tear on the engine during an extremely cold start and warm-up. i.e., just because it starts does not mean the use of a block heater is not a good idea.