"Kathy Dahl-Bredine developed the Windshield Shade Solar Funnel Cooker while experimenting with various designs of cookers to introduce in the indigenous communities where Kathy lives and works in southern Mexico. She hit upon an utterly simple way to make an instant portable solar oven. Taking a reflective accordion-folded car windshield shade, you can turn it into a version of the solar funnel simply by attaching little Velcro tabs along the long notched side. Here’s how:"
Also worth checking out is a post entitled "How to Make an Easy Solar Cooker...with materials you probably already have at home" by Our Twenty Minute Kitchen Garden. This looks so easy! I'm going to give it a try.