We know a ceramic blanket makes the best heat riser insulation.
So why not use the blanket AS the heat riser?
Yes, it may not be rigid enough by itself.
What about soaking it in a wet clay slip. Or maybe use fireplace cement.
Then wrap it around a form, possibly cardboard, of desired length & diameter.
Then use dry blanket as additional insulation.
Burn it out & attach it to your core.
This seems to me to be the best possible heat riser one can make.