try to have a box, preferebly a carton one, wide enough for the m to walk around. then put an incandescent bulb or two inside that will be of the chicks level from the floor, u can turn the light off when there is enough heat in the air. try to cover the bulb/s with chicken wire to leave the bulb protected and the chicks as well. after they have more new big feathers i think after a week, transfer them to a wider pen. with the same heating source only during cold times or during the night.
this is a traditional one which is great. hope this helps.