Make parabolic curve
Design a parabolic curve choosing focal length between 5" and 10". Once a template is designed, it can be used for several other cookers. Keeping in mind that the curve is straight and perfectly centered, have it traced on the open side of the box.
Cut out the curve
With the help of a utility knife, cut out the curve ensuring that you are exact and accurate. Now cut a perfectly measured piece of poster board that will fit exactly on the curve which is already in place. Carefully attach the cardboard piece working carefully at the edges and center.
Paste the Aluminum foil
Apply some white glue at the curve and then paste Aluminum foil over it with the shiny side exposed. Try to keep the foil as smooth as possible by pressing in the middle and then to rest of the area.
Fix the cardboard supports
Tape two scraps of cardboard as supports onto each side, exactly opposite each other. Test the focal point with the help of sun or a projector light and look for a bright spot formed due to concentration of light.