I am building my first reverse flow and I am looking for ideas for my firebox door. Here is a pic of the firebox. My build is a 24x60 tank with a 24x20x24 firebox.
Just use a flat plate of 1/4" or 3/8" A36, use some 1/2" 0r 3/4" angle and frame in the top and sides of the box to give it more seal. I recomend double plating the top of the firebox with a small air gap between the plates .