A few suggestions

[Ken S]

Regarding the Super Isol, or Calcium silicate board. You can paint it with waterglass(sodium silicate). Available commercially as Rigidizer. This is a waterproofer. You can then set your floor super-isol over clay/sand and have it be level and stable. Also when building your ovens the first row should be cut at the angle of your vault. This will minimize the gap on the outside of your bricks, giving you a more stable and consistant rise.