Adjust the flap in your pizza box oven so that the sunlight is reflected more strongly onto your food. As the afternoon passes and the sun moves, you will need to move your oven flap repeatedly to reflect the most amount of radiation into your oven.
Build flaps onto the sides of your oven top flap. Cut out pieces of cardboard to form side wings. Tape them to either side of the top flap. Cover the wings with flat foil to capture more sunlight and reflect it into your oven opening.
Tape mirrors onto the surface of your oven top flap and side wings. Mirrors have a smoother surface than foil, so they will reflect more of the solar radiation into the oven instead of having the light diffusing over the entire surface of the box.