The commercial cable release I used contains a single switch, so there is no "half pressed" focus confirmation like you get with the camera's shutter release button. Commercial two-stage push switches are uncommon and quite expensive, so I decided to make my own.
I used two sub-miniature push buttons operated by a trigger made from an old toothbrush handle. A small piece of plastic foam pushes one of the buttons first, then compresses to allow the other button to be pressed. The picture above shows it during construction, before bending the aluminium mounting plate to fit the shoulder brace.
A cheap audio cable with a 2.5mm right-angled jack plug is used to connect the switches to the camera. As shown below, there is just enough room beneath my 500mm mirror lens to accommodate the thickness of the mounting plate.
I haven't yet decided if the trigger needs to be shorter. There's plenty of room behind it for the rest of my fingers when using my index finger to operate it.
