The best way to get something ergonomic is to mould it out of plasticine and see how it feels in your hand
. What you ideally want is something that "feels natural" in your hands. Dont forget the balance too (too top heavy not good).
I'd thin it out if possible, make sure there are no sharp corners and maybe make some rubberised grip (although the last bit is harder). Perhaps the display could be higher and the grip thinner near the bottom? Maybe just optimise the "width". Seems the thumb rests on a corner that looks fairly sharp. Ideally you'd want the thumb where the lcd is though.. (well, further down). See how you hold a highlighter for example.
Though, you'll want to keep it symmetric so it can be used left and right handed (I assume you can flip the display text for that?).
Although, if the case isnt as ergonomic as it could be, at least if it went into production you could get some going and then keep a fancier case for V2? It may just be easier to add a grip on top and bottom so you can "pinch" the probe similar to what you're doing in the pic but rotated a little so thumb is on the flat, not the corner. Just doesnt feel as ergonomic to me as wrapping the thumb around and having the screen higher up (I was playing around with a highlighter).
I really have no idea if i'm making any sense or being even vaguely useful