For a while, I used an Ergodex gaming keypad for my inputs. I designed a custom layout for the direction keys, the ESC, START, STOP, Pause, single step, and maybe another one or two. It was real easy to program using the simulated keystrokes. I really liked the convenience. I could start/pause/stop/ at will without having to reach for the keyboard or mouse.
However, I ran in to trouble a few times. Even though the START button was not immediately at the edge, there were a few times it was hit accidentally. Guess what - the machine started. Fortunately, my fingers were not too close to the cutter. Now I use the Tormach shuttle and the keyboard for starts and stops
Whatever you do, carefully consider what would happen if you dropped your device, it got splashed, something fell on it, a child came by and pushed the pretty button, and everything else that might remotely happen before you finally to go with another START button.
John