Use only non-flammable materials, like galvanized metal.
I STRONGLY DO NOT recommend the use of foil backed foam board. The foam is designed for use inside wall structures and is not designed to be exposed to open flame - it releases toxic (deadly) fumes when it burns. Additionally, you usually end up having to build a wood support frame to hold it, while galvanized can be pretty much self supporting if properly constructed.
Cement board is way too heavy and difficult to cut and shape.