Now that the furnace is cold, I’ve had a good look at the lid. In addition to trimming the bricks back to 13.5 x 13.5 inches, I need to replace the metal strip holding the bricks in place.
The original plans from Mark required 4in galvanized sheet metal flashing, which I could not find. I took aluminum flashing and cut it into 4in strips, and that has worked fairly well for years. I’ve replaced parts of it at least once when the tightening mechanism tore the thin aluminum, but it seems to need replacement again as it’s been deformed by the too-large lid.
This time I plan to obtain some 4in sheet metal, somewhere between 24ga and 30ga, with me leaning towards 24-26 ga at the moment. As I cannot source a roll of 4in sheet metal flashing anywhere, I will try Metal Supermarket and see if they will cut me a few 4in slices off some sheet metal.