Raff, the foil cover on the meat will be a nice shield against ZnO powder falling on your meat but will also be a major radiative heat shield as well. (Like those metal plates around a car's catalytic converter.) The thermal design of that cooker is a bit unique. With the heat source ON TOP, it's "fighting gravity" so normal convective hot air currents aren't working in your favor. The main way the thermal energy is transferred to the meat is through radiation, so sticking in any additional metal shield before the meat will greatly slow down your cooking times. You may want to consider doing a "dry run" with no meat and really pile on the coals. The inside galvanized surface will almost certainly be white-to-grey powdery. That's ZnO. Take it in to a sandblaster and get it all removed down to the bare steel. You'll need to religiously keep it coated with cooking oil after that (it will want to rust) but at least your heavy metal concerns are now trapped in someone's sandbox.