I'm in the process of installing an RSF Opel3 zero-clearance fireplace in my new home. I have sheetrocked the interior of the cavity in which the fireplace will sit with 5/8" fire-rated drywall per our mechanical inspectors request (can't find this in the codebook, but I am going to honor his request). Now, I am wondering if I should install Durarock cement board on the floor of the cavity for the unit to sit on versus just sitting on the OSB. I currently have the unit sitting directly on the OSB with the required spark guard sheet metal flashing in place. Installing the Durarock would require me to pull the unit out of the cavity and custom cut a sheet of Durarock to fit in. Any thoughts?