I know how you feel. I have had box turtles for 20 years and I still get nervous when the temps drop below freezing. I agree with the first post as I also pile on the leaves and compost over my hibernating turtles. I go as far as to mow my yard in the late fall just to get the clippings and you can also rake the yard or gather leaves to pile over the turtles. When it gets into the teens, wich is rather rare for the dallas area that I live in I will throw an old towel over them for added measure. It might be overkill but I error on the side of better safe then sorry.

gather those leaves and grass