Looking for input

We are just beginning our studies to see how best to construct our rocket mass heater for our greenhouse. I came across this video and it used some materials that I haven't found used yet. Any thought on these being good choices? Thank you for your help.

Everything looks good for a demonstration model, but for one that you put in your house, you need to assume the inner metal part of the heat riser will burn away. Instead of using a loose fill like he did, I would recommend using a clay sand mud mix with an insulating ingredient like vermiculite. When they make brick, they mix clay and 25% to 30% sand (Just like in cob) and fire it at about 1800 degrees. This means that you will be transforming the mud you put into your heat riser into an insulating brick material. It may crack, but by the time the inner metal wall of your heat riser burns away, the now brick heat riser should hold together with some help from the outer metal skin. I recommend making you heat riser at least 3 inches thick though. It will help it hold together even if it cracks.