Measure your flash head with a ruler. Cut a hole at the bottom of your bowl equal to your flash head. Test the hole to confirm it fits your flash head.
Cover the bowls exterior with newspaper. Spray the interior lightly with silver spray for plastics. Ensure the paint is smooth and thin. Leave the paint to dry. Alternatively, tape the interior with aluminum tape.
Fold the cardboard into a rectangle to fit your flash head. Tape it around the flash head.
Cut your cardboard along the edges. Fold out the untaped part.
Mount your bowl on the flash head. Fit the hole in the bowl head into the flash head. Glue the foldouts and carefully stick them to the bowl. Tape the foldouts to the bowl to hold it firmly.