Brick riser insulation ?'s

Hello, i have a 6 in system using a 55 gal barrel being built
I am about to insulate the brick riser
Im thinking of hardware mesh form packed with ash and perlite
Is there a proper ratio?

Any other natural material suggestions for replacng the perlite?

Is the coarseness of the air space between the barrel and insulating jacket going to be an issue?
Does this airpath need to be smooth?
Should i use sheet metal or concrete forms?

Does a concrete form get removed before placing barrel is used?