Mounting a Starglo SG6 NG fired ceramic infrared heater under my porch roof -- using cement board attached to rafters

I’ve been using a succession of pedestal NG heaters for years (they wear out). I’d like to mount a Sunstar ceramic radiant heater (twin burner – 65,000 btu) beneath the roof (12 pitch, 20′ floor to peak).

The manufacturer specifies a minimum distance to combustibles of 49" from top and sides of the fixture.

Question, would I meet fire code by affixing 1/4" cement board to the rafters? If so, what would the minimum distance become?

https://sunstarheaters.com/product/sg-series/