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They certainly don’t “create” heat, because they aren’t a heat source. They’d definitely restrict heat leaving through the lens and possibly the top part of the head / bezel though. Although if the heat level is that high, my main concern would be melting the diffuser 
As for shattering the lens, I’d most definitely doubt it. I’ve run torches on turbo in falling snow and the temperature differential between it landing on a hot lens has never cracked a one. So I’d doubt heat build-up under a diffuser would crack one. In saying that, I’d never say never from my life experiences.
what light and what battery ? light is probably ok as diffuser just builds up more heat i guess