The usual mixture a long time ago was 1/3 loam, 1/3 sand, 1/3 dirt. What you want is something that holds water, but drains excess water.
Try mixing up sand, dirt, and organic stuff (I use used coffee grounds) starting about 1/3 each, and see how it drains. If you pour water on it and water pours out of the drain hole in the pot, it should be OK. It should hole water for a day unless it is pretty hot, and be close to dry the next day.
Once you get the right proportions, put the ficus in it, then watch to see if the soil mixture clumps up and water doesn't drain. If that happens you should have used more sand.
Depending on the tropical island, you may have lava available in small pieces. If so, use that in place of sand. "Small" means 1/2 centimeter or so.