I might be at my boat later today. I will take a picture of my 2QM20 throttle area. Which might help you design your custom made bracket. I expect the setup will be very similar to your 3QM30.
If not today, will have wait until Friday or Saturday.
I don't recall exactly how mine was configured when I bought the boat. I think that between the throttle arm and the bracket was a small spring. It wasn't robust enough for the purpose, so I replaced it with one that is stiffer. The idea is that when I throttle back down to idle with the pedestal throttle lever, I can feel the spring's resistance. And then know not to "throttle down" any further. Further back equals stop engine due to complete shut-off the fuel supply. If I want to shut down, I pull the pedestal throttle lever past the spring's initial resistance all the way back, and hold it there until the engine stops due to fuel starvation. It takes just a few seconds for the engine to stop. I hold the throttle back for a few more seconds just to be sure that any residual rotation won't cause the engine to come alive again when the spring returns the lever to the idle position!
Your engine looks pristine. Is it connected yet to the pedestal?