Slice the oranges in half horizontally. Using a citrus press or juicer, gently juice the oranges without damaging the peels. You will need 375 ml (1 1/2 cups) juice. Top up with store-bought orange juice as needed. With a spoon or melon baller, carefully remove the pulp. Arrange the 8 hollowed orange halves on the foil, taking care to make them stable and level.