I'm using foil (or aluminum sheet, haven't decided - need to finish fixing two doors first), but I'm going with MDF to avoid any swelling of the wood.Made a few. At the BORGs they have a shield material for the insides of them. It's basically 1/8" bubble wrap with foil on both faces. It has 98% reflectivity of heat so it pushes it all out the front and protects the wood from the heat. Sold by the roll, but I can't remember how much. Found in the insulation section.