You REALLY need to make sure you have the angle of the sun directly perfect. You could use ether mirrors or aluminum. That's how I made my solar furnace science project. It cooked an egg!
-OR-
To design a very useful solar furnace visit a site in where it describes a "Curnutt Solar Furnace", his design incorporates a base cemented into the ground which then is loosened and pivoted to follow the direction of the sun. There are many mirrors focused on a central point far above the main design, which actually creates steam and incredibly high temperatures. A good site to check out is http://www.phoenixnavigation.com/ptbc/articles/ptbc36.htm
-Hope this helps!!