I think you may be referring to the heating pad Cave, where you shape wire in a hump and put a heating pad on it that does not go off automatically. The chicks use it when they need it. Blooie has a thread about that.
You might want to cover that top panel of glass with hardware cloth. I have had hawks break glass twice trying to get at the chickens. One killed my rooster. The second time i had wire on the inside so no chickens were killed.