Ok SMF followers. I need some advise. Ive aquired some 20 inch pipe, a little over a quarter inch thick. I have a piece 5 feet long and would like to turn it into a reverse flow smoker. I want to build my fire box out of plate steel. How big of a firebox do i need and how big do i make the opening into the pipe. I thought i saw a web site on here a while back that gave all that info but im not sure. Any suggestions are appreciated.