After watching the video on why the conversions are not as efficient I am thinking that if the fire tube is insulated with perlite and then a thicker top or a fire plate above the top, the old fire box should work the same as a 55 gallon barrel does. I have built one out of 4 inch square tubing 1/4inch thick, I know over kill lol but it is some that I had laying around and then put it in an old job box that I had laying around. Still have not insulated around the burn area but so far has been working great on the test burns. Definitely very little heat from the flue pipe or in the flue pipe it self and the box it self has gotten up to 200 degrees on the top. The reason I am wanting to use the fire box as a heat collector it that it has the outside housing around it to reduce the chance of severe burns.