Justin’s modification of the long WK tube seems good, but one has to be very observant and “easy-going” in the beginning, until the “crust” surface gets strong.
For Imbert hearths, I see no need for “foregin” insulating materials inside the hearth.
Outside the air-mantel around the hearth there is a good motivation to use proper insulation, if the ready gases are drawn “through” it,
or outside the surrounding original ready-gas mantel.
Good luck on your “site” and elsewhere!