BUILD YOUR OWN SOLAR COOKER - BEST DESIGN I'VE SEEN

     More pics

http://www.angelfire.com/80s/shobhapardeshi/ParvatiCooker.html 

The above is an example - not quite how mine will look. Mine has 12 flat sides which can be made from old cardboard boxes.
Full details and pictures will follow as soon as I have them. Meantime, click here for TEMPLATES. for the NEW DESIGN and follow instructions below combined with reading the link above.

Print out templates. Cut them out and draw round them onto your cardboard box sides.
Cut 
- 12 pieces of  A1
- 12 pieces of A2 
- 1 Base B 

Cut A1 with corrugated cardboard 'internal ribs' running from top to bottom.
Cut A2 with corrugated cardboard 'internal ribs' in line with the tabs (side to side) so that the dotted lines will fold easily.
Cut 24 strips 50mm wide with slight fold in middle to join each section together: 12 x 235 long 12 x 180 long Corrugations running length way. This is important so that they will crease in half easily.

Glue all pieces A1 and A2 together using the tab on A2 behind A1. 
Crease all joining strips down the middle and fold slightly.
Glue the longer 50mm strips behind A1's
Glue the shorter 50mm strips behind A2's
Glue in base B
Cut tin foil in sections matching the templates and glue each one in place. Use PVA wood glue. Mix it with a little water so it's easier to apply with a brush, but not too runny. Paste it onto the card and while still wet apply the tin foil. Smooth it with finger and use a damp soft cloth to wipe of any excess glue while it is still wet. Wipe and polish the surface now, as you go, to remove all excess glue. This is to ensure maximum reflection of the suns rays.

Refer to website link above for further details.

Below is another different link to a different model solar cooker: