I finally have a picture of the wonder box or wonder oven. It cooks with retained heat. This green one is a standard size wonder box.
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This is a picture of a wonder box put inside a duffle bag type thing, for a more portable version. See how the pot is placed inside?
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Wonder boxes are good for hot and cold as seen here with the ice cream.
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I am still tracking down the pattern, but I have the information of a gal down in Elk Grove that makes and sells them for $35.