If I'm following correctly, putting refractory up the sides of the firebox may not be helpful. My firebox does accumulate some creosote, which burns off enough so that so far, at least no cleaning needed. The bottom of the firebox is refractory, and stays clean due to the heat below in the refractory tunnel. Manual for the Tarm states that if (unusual situation) creosote accumulates on the refractory on the bottom of the firebox, it needs to be cleaned off. The firebox obviously is much cooler than the gasification tunnel. Its purpose is to maintain combustion to drive gases into the gasification tunnel. I supose a hotter firebox wouldn't hurt, but I'm not sure it helps.