If you are concerned then perhaps use split brick and wrap that with ceramic fibre?
How do you intend to close the top of the bell or clean out the floor ash?
Personally I would want to have access to the core and riser.
Delicate in this context means the coating has been baked hard to the extent of being fragile, it is possible to crack it by pushing it with a finger.
You dont have to coat the fibre with anything and it should last for years but, it will by then have become a hazardous material and potentially deadly if breathed in!
So ‘delicate’ also means to me, that I have to be very careful not to break the coating that is containing the danger, but delicate does not mean the riser would not still Function.