In this picture you can see the rudder quadrant and the Starboard shim I made to attach the rudder position sensor arm to. Starboard is a high modulus engineered plastic sheet product available in thicknesses to 3/4 inch. Starboard is impervious to moisture and rot and is very well suited for use on a boat that is why I chose to use it. It works, cuts and taps similarly to wood and is a tremendously useful but expensive product.
On many Edson steering quadrants there are two 5/16 threaded holes already drilled and tapped into its surface. I simply measured the center to center of these 5/16 threaded holes and transferred the measurements to the Starboard shim using a set of calipers. I then drilled the two holes and mounted the ball (the arm has a socket and the base is a ball) to the Starboard perfectly centered between the two 5/16 holes. With the ball finally mounted, and screwed into the Starboard shim, I then bolted the shim to the quadrant. In the picture you can see the two 5/16 bolt heads with the ball mount for the sensor in between them.