Just a thought-- some of those boards might have air pockets that would expand and possibly explode under heat. "Wonderboard" has styrofoam bulking particles. A sheet of galvanized sheet metal may work better, but I'll defer to your experience.
I’m just a ham and egger. I’ve replaced a water heater or two, plumbed a new bathroom, and done misc repairs.
To bad you can’t grab a scrap asbestos shingle any more. That would be perfect.