[aerohead]

Years ago, I thought it was generally agreed that, the 10mm Coroplast, from The Home Depot had turned out to be more than adequate for belly pans.
It's a thermoplastic, and will 'yield' at some temperature threshold, but exhaust components can be insulated, or heat-shields fabricated to mitigate heat effects.
Sag testing will reveal necessary center-to-center support spacing.
And hat-section stiffeners can be attached wherever deemed beneficial.
Consider 'sections' of pan, which can readily be removed for inspection.