Heatproof shelters aren't solving the problem. If you assume the hypothetical impact on well-being to a billion human beings, and potential deaths of 100-300 million human beings, what kind of physical scale are we looking at to shelter a billion human beings assuming one shelter every x km? It's about on a par with only equipping a large commercial airliner with a single oxygen mask.

How about the materials required? The scale of manufacturing? The impact of the manufacturing on the environment? The impact of shipping and installing them on the environment?

It would sooner make sense to create underground cities, but that's no small endeavour.