I prefer to use a pair of combo scissor clamps. This is what I usually use here but I also have some of these that work well too. I have always eyed the Ketchum Release tool and thought it looks pretty slick (not Dr Slick) but would work well. However the price is ridiculous. Twenty bucks for that little piece of....well plastic? Highway robbery in my opinion.
I assume here is the item pszy22 modeled his tool from and how it works. I'm not at all a fan of hanging a fish by the hook out of the water but this is how it works. I guess if you dunked your hand under the surface and held the fish above in the water column this will work without taking the fish out of the water.