It's a homemade refractory cement made with Portland cement, fire clay and perlite. There was a top with a hole in the middle also that was not on in the photo made of the same stuff all of which I let dry for a week before using. The refractory only lasts a dozen firings or so before you have to replace it but it's cheep. That's not a heat gun it's a blower that I used dog food cans with both ends cut off to hook to the furnace
I like trying to use whatever I have at hand in my projects spending as little as possible and most of the parts come from my collection of salvaged items