You will need:
A large rectangular piece of glass, such as a discarded storm window
A large rectangular wooden or cardboard box
A smaller box, about 18 by 22 inches, that will fit easily into the larger box
Six small spacer blocks (available at the hardware store)
Wadded insulation such as crumpled newspaper or construction paper. (Do not use Styrofoam peanuts as they emit toxic fumes)
Aluminum foil such as Reynolds Wrap
String, such as used in packaging
Non-toxic glue or tape other than duct tape (also toxic when heated)
Cover the bottom of the large box with the insulation, placing the spacer blocks next to each other on the bottom, in order to balance the smaller box. Place the smaller box inside, filling all space between the sides of the two boxes with more insulation.
If the larger box is cardboard, and has flaps, fold these over to cover the insulated space, if the box is wooden, find folded cardboard and cover the insulated space, as well the inside of the inside box, and tie it with string at the corners.
Line the inner box and inside of the cardboard sides with aluminum foil adhered with non-toxic glue so that the entire inside of the cooker is foil lined. Set a piece of square glass over the top, which will reflect the sun. Make a cardboard lid with a foil lined reflector flap cut out of it that it can be adjusted to catch the sun.
When cooking with pots, use those with thin metal, and if you can, paint the pots black, and set them on a black-painted tray to absorb more heat. Use black latex paint. After the sun goes down, preserve heat with bricks on top of a blanket over the mirror lid.
Generally, you can count on good cooking time if you place the solar heater directly under the sun, preheating it half an hour before baking cakes or pastries.