While darkened cardboard or other non-metallic materials work, a black sheet metal tray works best. One important innovation is to elevate the tray slightly with small pieces of cardboard underneath so that heat gathered by the tray is not drained out of the bottom of the oven. If you are short of foil, then use some other locally available material, such as feathers, wool, etc. While some cooking can be done in any non-reflective pot, ones made from thin, black metal work best. Cast iron works but its weight can delay cooking or inhibit it completely during marginal conditions.
Shallow pans cook better than deep ones.
The single biggest reason for failure in solar cooking is not putting in the food early enough in the day. Foods take longer to cook when there is smog or haze. Since the clarity of the air can make as big a difference as the sun angle, learn to check out the color of the sky and adjust time accordingly. At times the sun may be shining, but the sky is white, indicating that smog or haze are present. A blue sky indicates clear air.