Here is how I grill with banana leaves:
I cut the leaves about 4" longer than the fish, wash them real good with water, sprinkle the fish with salt and pepper, and what ever spices you like, inside and out, maybe a little lemon or lime juice. Heat the leaves on the grill for 15-30 seconds (to make them pliable), place the fish on one half of the top of the leaf next to the spine of the leaf and fold the other half over and tuck under the fish, fold the second half over and hold together with toothpicks, fold the 2" at each end over and hold with toothpicks and cook on the grill until it's 145° in the thickest part.
Here is a spice rub that I use that is good on meka, tuna and mahi:
1/2 tsp annatto seeds, 1/2 tsp black peppercorns, 2 allspice berries, 2 cloves, 1" piece of cinnamon, 1/2 tsp dried oregano, 1 dried bay leaf, grind in spice grinder and sprinkle on fish.
Enjoy and post some pics,