Following his departure from Egyptian service and starting Sir M. MacDonald & Partners, he continued his work on Egyptian irrigation schemes. The firm was invited to submit proposals for a second heightening of the Aswan Dam, and the work was carried out between 1929 and 1933. About the same time, MacDonald was responsible for work undertaken to strengthen the Esna Barrage, and he advised on various schemes for land drainage and irrigation in Greece and Portugal. After World War II, he was again consulted by the Government of Egypt about the Aswan Dam, and submitted a report advising that a third heightening was practicable, though never carried out.