The thing to remember is that in this part of the world, the soil temperature never gets much below 50F. So the main trick is insulation. If not exposed to the low temperature air, the ground will keep things pretty warm. Plastic sheeting is OK, but I'd throw a thick blanket over the whole thing on a cold night. With regard to xmas lights, pay attention to wattage. These days, with LED lights, they consume, and emit, a paltry amount of power. Those won't keep anyone warm. I stick a 60W incandescent under a box over my frost-intolerant plants on nights of a hard freeze.