In 1932, if you detached Southern Lebanon, (South Governorate and Nabatieh Governorate, but with Jezzine District staying with Lebanon) as an Independent State, or a part of Syria and made Tripoli a Free City under French Control, the rest of Lebanon would be nearly 80% Christian and Maronites would have a majority.
That seems the easiest way to make Lebanon solidly Christian, and it creates a viable Lebanon (though if there are concerns about viability, Sidon could be added, and it won't shift the demographic picture much, only decreasing the Christian percentage about 3 or 4 percent).
The Druze, Shia, and Sunni would all be at roughly 7% (give or take a percent or two) in this scenario.