Here's the brooder I built. It's a 4'x4'x2' box elevated 2' above ground level. I built it with a divider in the middle so small chicks wouldn't get lost and not find their way back to the heater. Once they grew bigger, I could simply remove the divider giving them full run of the space available. The first batch to go into the new brooder consisted of 15 BR chicks... which quickly outgrew the 2'x4' space they were in. By the time they were 4 weeks old, I had another batch of 7 chicks in need of a brooder. I built a heating pad cave for the older chicks, and moved them to the other side of the brooder, and put the new chicks on the side with the Sweeter Heater. This worked great for about a week, and by then, the older chicks were getting really cramped. I built another brooder box on top (back half) of the existing brooder. Once I moved the babies and the Sweeter Heater to the top brooder, I removed the center partition from the lower brooder... which doubled the space for the 15 older chicks.
The original brooder and divider with two sets of chicks. A hard-wired timer to control the lights is mounted on the top.
I needed more space, but didn't want to build a separate brooder... so I went up top with the addition.