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Some years back I experimented with block heaters. The Honda OEM block heater I was using was 450w, which helped, but I also put a pad heater on the oil pan and transmission, which made a noticeable improvement. I got these from eBay and connected them to a multi-outlet that hung from the front grille.
You really have two separate issues -
1) Getting enough watts to heat the engine block while it's sitting - you're not going to be able to insulate it enough to massively change heat retention.
2) Closing the bay off enough from airflow while the car is moving.
While the car is sitting, air in the engine bay is essentially still. While the car is moving, air might be passing over the engine at 100kph (or more, because the grille and ducting may accelerate the air).
Heat rise will not be a significant factor in an engine bay. It's leaky enough that any warm air will be blown out by the slightest breath of wind, and while moving, there will be no thermal gradient. Hot spots are generally from heat sources, and what that direct radiant heat hits.
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