[bojan]

After some more experimentation, I came up with this solution (no springs).
The focus is stable now (focus ring is held in place with nylon screws on additional ring), it is easy to adjust it (the OTA itself is fixed properly as well) and the focus ring doesn't move by itself - so it stays OK even from the last night session.

One more detail - I replaced balls in bearing with nylon tubing, 3.5mm in diameter.. the original balls were actually too small (3mm - probably used by mistake, or the rail was not made up to specification) so they rattled inside the bearing. Now the whole structure is rock-solid.