Bring a large pot of unsalted water to a boil.
In a large bowl, combine the grated sweet potato and coconut. Mix in the sugar, butter, cinnamon, vanilla, nutmeg, and raisins.
Mix in just enough flour to form a cohesive dough. If it is too dry, add about 1/4 cup water.
Cut the banana leaves into squares, 6-10 inches depending on how large you want your Ducana. Rinse the banana leaves under hot water or heat lightly over an open flame until they are pliable. Transfer some of the dough into the center of a banana square and form into a rectangle/cylinder. Fold the banana over the dough to cover on all sides, then wrap with the twine to secure or wrap with foil. Repeat with remaining dough.
Reduce the boiling water to medium-high for a gentle boil. Add the packets of dough and cook to desired doneness, 30 minutes for slightly soft and 45 minutes for more firm. Remove from the pot and allow to cool for 10 minutes before unwrapping.