Double Covering in Ovens

Question

K'vod HaRav: 1) Where does the practice of double covering foods when placed in non-kosher ovens come from? To my knowledge, is not recorded in the Shulchan Aruch or classical guides to halacha. It is not even found in many modern books on kashrut. 2) Does double cover even help when suggested, or is a single cover enough? Many thanks, JB