I always used cardboard boxes until my son built me a big brooder. They are easy to keep clean when they get dirty just toss it out and get another larger one. Sawdust on the floor isn't a good idea. They will peck at it and eat it. Newborns should be on papertowels or like I use for the first days old pillow cases. I sprinkle the feed around and they get the idea of what to eat. I also use a 100 watt bulb It gives off plenty of heat when its inside the house. I usually clamp the light onto the side of the box, but I do have some my son attached to strips of wood You can hang the light that way and as they grow you can adjust the height of the light by turning the wood lenghwise and attaching it to the side of the box.
Sawdust I have three big bags someone gave me but I am afraid to use it as it came from plywood. Plywood has glue in it and I am afraid it might harm the chickens.