Gotta go with free cardboard as well. I was going to build, but decided to go with a box. I have a box that a stove or dryer comes in for my brooder. I didn't even put a lid on it. No problems whatsoever. My theory is that unless you're brooding multiple batches of chicks all the time, why have a brooder laying around the garage until next year, when you can burn it and start over with a clean, disease free one next year??