in an emergency i used a cardboard box and ceramic heat buld, the sides of the box were to tall and the chicks all died, overheated. walls of box only need to be 9 inches tall and as big as poss so they can come to the heat when required and move away when warm, the heat source needs to be higher than the box. you dont need a themoeter as you can watch the chicks, if all huddled under the lamp lower it, if all around the perimetre (to hot) raise it. they will move in and out the same as they would if they had a mother.