After experimenting with different postures behind the eyepiece, I used a convenient location for a brass bolt (3/16") with the wide head and some steel washers for the spacer to install the ball in the convenient place suitable for work behind the eyepiece, as well as from the knees behind the Telrad. There is only one hex nut on the inside of the OTA, so the ball has some freedom of motion around its position as it is being pushed or pulled around. It is also rotating around that axis with some friction. At first, I thought about fixing that with another nut on the outside, but after the thorough field testing decided to keep it rocking free as it is now.
I've realized that such a feature is actually helping me with faint motions of the OTA while star tracking, especially by both axes of the Dob's mount, making slow motions smoother and with less overshooting (compared to moving the OTA by its rim). I have mastered the special two fingers grasp of the ball for that which allows extremely fine two axes tracking with that ball.
- Replace the outside metal washer to some soft material though in order to prevent scratches (done, 3d printed)