I guess there is a direct relatinship btw insulation and chrcoal durability in those designs, therefore good joints must to be setted, and a good chimney design too, for the smoke not to return. Other thing I imagine is the bottom (or floor) of the forge some concave for coal sets downing while is burning and avoid lost time trying to get the amount reorganized- And the most important, to be ergonomics, so your back is not to bend too much for looking inside the fire. I wish my new forgw to be such like.