Usually, the hearth plate is a sacrificial item, used to protect the underlying furnace brick (and elements). Typically a furnace bedding powder (like bone ash or magnesia) would be placed on top to absorb spills:
www.lmine.com/category/furnace_bedding.html Adding another plate to protect the inexpensive hearth plate seems redundant. Depending on the size, hearth plates are quite low cost. For instance, this plate which is 18x11 inches is under $30:
www.lmine.com/product/26460.html