Original web site with this article:
http://solarcooking.org/funnel.htm
[Image: 01.jpg -- Photo description this image shows a cardboard
box being used to support a silvery funnel, roughly 3 feet high.
The funnel is placed within the box so that the sides of the box
hold the funnel upright. Sitting within the base of the funnel
is a black jar inside a clear plastic bag. The plastic bag is
inflated like a balloon to seal an insulating air pocket around
the jar.]