Yes, I use a heat mat with a thermostat under my brooder box. The temperature control is not precise but adequate for chicks at that age. If you put the mat under half of their box they can move to where they are most comfortable
I've never actually used one but they're said to be very good for this purpose. The temperature is more appropriate than what you're likely to get from a heating pad made for humans, and unlike the human heating pads they don't automatically shut off after a certain period of time.