I've cooked fish in foil lots of times, mostly skinless fillets, and this year I intend to try it on the barbie. Last year I bought up, in a sale, a supply of special foil bags with little windows in, big enough to fit enough fish for two. The fish goes in with some butter, seasoning, a sprig of lemon thyme and a squeeze of lemon juice, maybe a hint of fresh garlic (you could steep a bashed clove in a spoonful of olive oil and just put the oil in the bag, though I still want the fat to be mostly butter), then you fold the bag shut. It's about 20 minutes in the oven but it should be a bit less on the barbie. I've cooked tuna steaks on the bars but most other fish tends to fall through. I have a cast iron plate that fits over half of the grill on my Weber 2200 barbecue, but it's the devil to clean after cooking fish on it.