would there be a problem with building a form to create my own arched base for the flue crock. I was thinking of using the Heat Stop 50 mortar and some wire fence material inside a plywood form to make a base to set the flue on. I am thinking to make it 3" thick and of course with a 8 inch hole in the middle. would the mortar hold up or would it crumble? should I add some fire clay? or use concrete alone? Help!