It is easy to cook with concentrated solar light. You need however to wear sun glasses or you can use a paper mask with two holes to protect you from strong sun light.
Some important things:
1. You can use aluminum membrane that is used by builders on roofs. They put it below roof tiles - it is really easy to work with this material. It is strong and you can make different forms. You can find it in building supermarkets as Mr. Bricolage or Practiker.
2. You will need a glass container and then you put your pot or stew pan with the meal inside the glass container. It will provide high enough temperature.
3. The best design in my experiments was conical design.