i have two different sized fish cages (rigid), and a flexible oblong basket that conforms to the items enclosed (not fish-specific, so good with veggies, bread, etc.).
the fish cage handles are metal with silicone and wood, respectively. they stick out, but that just leaves a small gap between the grill and the top.
the flexi basket is neat, and makes turning unwieldy individual items a breeze. it would work for a fish filet, but not so much for a large whole fish. i got it at williams-sonoma on sale, but have seen them elsewhere for about the same sale price.
note: individual filets will do better with finer metal meshes, and i think the larger whole fish will do better with fewer but larger metal wires. the basket size will determine which size fish you can and -- esp. in the case of the large rigid cages -- must use (because smaller fish won't be secure within a too-big cage). that said, a mesh-type cage (or one with closer wires) will let you put other ingredients (aromatics, e.g.,) around the fish, and they won't "fall through the cracks."
note, they are handy, but like broiler racks, are a pain to clean up. non-stick surfaces and pam help SOME. on the other hand, the non-stick will eventually corrode. so....all in all -- stick with real good stainless steel.
i like mark's idea that baskets can be used over a campfire (i.e., no grill grates).
Personally partial to the grill basket made by Rome (big fan of their pudgy pie irons and waffle irons, so a little biased). They make different shapes and sizes.
Spatula, no matter how big, won't save you if you are cooking delicate fish. A basket lets you load your fish up, stick in some veggies or a few slices of lemon, and then grill away. Works well over an open fire with no grate.
All you need to grill fish is a wide spatula. Maybe two working together. You don't want something that has handles because the handles will have serious issues with the lid- if you can fit the whole thing you run the risk of melting any stay-cool handle (like with my well-worn chimney starter), and if you have to leave the handle sticking out you can't put the lid back on right.
There is a simple way to keep fish from sticking to your grill. Just before you put the fish on the grill, dip a wad of paper towels in vegetable oil, hold the paper towels with tongs, and rub down the hot grill with the oil-soaked paper towels.