I've used those kinds of cases with the hand-holder on the back before. Not very comfortable, in my experience.
The "utensil" as you call it

might be hard to figure out just by looking at it. It's meant to be used with a "naked" Fire (though I suppose it could be used with some cases as well) and it's designed to be held in the position in this pic;
The curved area is for resting your thumb and gripping from below with the fingers. The notches are there so the utensil can double as a stand (like on the counter top when looking at a recipe, or on an airplane tray table). I'm not sure whether the notches will work as I expect them to or not though.
Note that the Fire Holder is designed to be ambidextrous and also to be used in either portrait or landscape mode. As it is now, it's about 3.5" long and 2.25" wide. And keep in mind this is just a rough idea and prototype stage right now, just looking for input about overall concept.