Yeah the K-26. My old design was using a 7 gal air tank in the same style as the propane tank forges here in IFI. The problem I ran into with that design was for some reason no one locally unless this post diggs someone up sells high temp/durability castable refractory. If it was my money to spend sure I would pay double to ship it, but not feasible currently. We have a few producers/distributors near here who say they do small quantity, but they either wont respond to me or stop talking to my part way through. So I switched to the firebrick box knowing its not durable, but we can easily replace it when we need to.
Thanks for the ITC info usually when I peruse through here I forget to check the dates when google brings me over so most of them about it must have been older. Plistex was my second go to, but I haven't heard of Matrikote at least that I remember ill go look into that.
I might still have enough Mizzou left over from making the burners that I can do a half face on the 7 gal with insawool 2in thick. with very rough estimate brings it to a chamber of 462in3.