I started my first oven with 28 ga. steel. It makes a bunch of noise while it is heating and some when cooling. But it does work. I have 2 inches of mineral wool. I had it up to 600 deg. Gets a little warm on the outside, but not very much.
As for the Hardi Board, one of the guys did his with that and then ripped it out and put in steel. Took to long to heat.
JT