I originally used John's hand held steering controller, but found it was not practical for a sit down balancing machine. You need something  very rigid to hang onto when riding. The machine does a good job of balancing if you don't fight it by trying  to balance it your self..
The stick is rigid in the fore and aft position , and will move side to side in the lateral position. a compression spring centers the stick.
Two set screws provide stops, and two more act as limit switches for the two micro switches that control the steering.
This could be done simpler by mounting the micro switched on the outside of the stick, but I wanted to have them hidden inside the stick.
I have access to a vertical mill and I never miss a chance to use it.  The micro switches are available from Radio Shack for 3 or 4 dollars.

The adjustable front training wheel serves two purposes. first it keeps you from pitching forward during any sudden stops, and secondly it limits the forward speed by limiting the pitch angle until you get comfortable with riding it. The rear wheel is fixed limiting reverse travel to a safe speed.