Do the 8-man treasure hunts. Seriously. If all 8 people have just 1 map each that's 12,800 gil MINIMUM per person. Chances are you'll get at least one rare chest with 5,000 gil in so that total will probably be higher. That's also completely ignoring the rare mats that come out of those treasure coffers...

I do 8-man maps with my FC every few days and each person has 2-4 maps to do... usually looking at 30-40k gil in about an hour plus a tonne of HQ mats.