My pit's chamber is ~40" and my plates are 1/4"T and 4"W. They work well. Anything in the range of dimensions mentioned will work. If you can get some thicker stock that will work too - plus it's adding more heat sink material to the pit as a bonus. Use whatever you can find cheapest and easiest. Don't worry whether it's cold roll or hot roll. Just clean any petroleum oil off of 'em and season with veggie oil.