For my rubbermaid brooder that I use when I get chicks delivered to my office or as an emergency chick hospital I have a flat heat pad meant for reptile tanks. I lay a folded towel onthe bottom of the box them the heat pad, then another towel. The chicks love it. They flatten themselves out to soak up the heat even if it's warm enough with the small heat lamp they don't need to. It can be disconcerting to see even if you're used to how chicks look dead when they sleep hard because on the pad they look melted. I found it's extremely effective for warm up shipped chicks. They can heat up and rest spread out without crowding under a lamp. This is NOT a recommended use for the pads so I don't recommend doing this if you are going to leave your brooder unattended. Heating pads for people can get tooooo hot and cook chicks, even if you set them on low. These reptile pads are at a fixed setting so they can't burn your fuzzies if there's a towel btween them and it.