Hi from Thailand...........
I've got leaks in my firebox in my new build and have been experimenting with locally available items and so far, not too successful. Living in Thailand makes it hard to get things that you guys in the US can get at your corner store or Home Depot.
My worst leak is where my stainless cabinet sits on a cement block firebox. I tried using strips of attic fiberglass batting and I think that the fiberglass is breaking down from the heat, so I will junk that idea and may try using a clay, but expect that to eventually crack. Meanwhile I found a link on this thread to bbqgaskets.com and sent them off an inquiry if they ship overseas.......expensive and long wait.
And I have another several leaks between the welded door and frame. Might fabricate my own gasket with RTV 750 degree [they have that here, amazingly], but not cheap.
BTW.....what kind of temps are we talking about in the firebox during a normal hot smoke @ -250f?? In a test burn, I had my cabinet up to over 500f at the top, but assume that the firebox being lower would be lower temps, as I have a straight up airflow. nd while we're talking about test burns, how hot should I get the temps during the first burns to burn off all the chems etc??