Allen: here is the link for the video showing how rockwool behaves as insulation on a heat riser.
I do not know how some people manage that the video appears directly here in the posted text.
What do you think about a heat riser like in the attached drawing? It is just an idea, I did not test it. It is cheap and should work fine. I am not sure the aluminum foil will last for long but something has to prevent mixing of the flue gas with the air inside the rockwool and save rockwool from being blown away.