hookway style rocket stove charcoal retort

So I have been doing lots of research on rocket stoves and mass heaters and I would like to start off with something basic and cheap, but useful. I would like to build a hookway style charcoal retort. Fore those that dont know this design, it is a kind of wood gasifier, rocket stove, retort hybrid.



https://ibb.co/album/fR3nYv

However, I would like to make some changes to the design to reduce cost and to learn basic rocket stove building principals to apply to bigger mass heater projects.

The biggest issue with this design I believe is the use of a metal heat riser. I have been trying to figure out a way to replace it with a cheap casted refractory riser. Its difficult, as you don't want the riser to be insulated as in a traditional rocket stove since you want the heat to apply to the wood surrounding the riser, it is then insulated on the exterior or the wood chamber. what would be the best way to cast a similar design out of refractories? Could I simply use steel but coat the interior with sodium silicate? Could I cast one out of portland cement and coat with sodium silicate? any suggestions and insights are much welcome and appreciated.

Do you have your heart set on a Hookway? They are nice but are complex I find and prone to the centre chimney receiving extreme heat stress. I prefer making my char in a tlud type kiln or my wood stove.
Gary Gilmore's Keystone is a nice simple build not as prone to metal breakdown due to extreme heat.

Here is a pellet based unit a little more refined but I prefer wood chip or wood pieces based units:


Cheers,  David

Cool thanks for the info. I'll do some reading after work today!