I just put a square of foil on the grill with the sides turned up and creased at the corners, place filets on aluminum foil, sprinkle with garlic salt, place a couple of rings of an onion on top and pour in some lemon juice. Once the filet is flaky, poke holes in the aluminum foil to drain. Allow the filet to continue cooking to desired dryness.
I scale my fish and filet them with the skin on for this. I like the flavor of the skin.