Now that's something I have thought of ... and that's making custom grips for my "working" pistols. Lee. Before we all had to go "standard," and were using .38's and .357's most of us had thumb rests (before combat ranges became popular) made to order for our individual hands. I would think that making fitting blanks, then using modeling clay to shape the handle might just work with the probe (baking the clay after it's made to fit). I haven't done that yet, but certainly have thought of it.
Bob