A sheet of aluminum plate when can even be obtained at a cheap price at the building store.
Aluminum is a poor choice. As far as metals go, it has very low strength and durability, it is highly malleable, it has a melting point well below temperatures reached in an average house fire, and it is very prone to corrosion. It offers very little over just using paper.
You can easily pick up stainless steel at a hardware store. Use this instead.