I was just trying to study the very same thing. Vermiculture is a hobby of mine picked up to help my herbs and marijuana. I understand above 90f will stress a worm, they prefer 70-80, and keeping it organic would be a must.
Everyone uses black plastic to gain heat, I was thinking about wrapping my 18 gallon tubs, with a white trash bag. This will reflect the heat and the light.
I suppose if I bury the 18 gallon half way or better into the ground, it will work as a heat sink. I hope it does and the worms don't leave through the drain holes I have in the tubs. Wow that would suck.