I use : a rod length of +/- 20 cm , than a 1/4" bearing ball , and the last 3-4 cm (clutch side) rod is made out of a 6.8 mm drill shaft . (6.8 drill bits are very common in a metric world ) This short piece of rod is made to a perfect sliding fit and can easily be removed to be ground to the correct length . It effectivly stops gear oil migrating . I have this setup long before DD's CRS was available .
It won't just fit in . The hole in the mainshaft is not machined . You have to hone and polish it to make a perfect fit . I think I used valve grinding paste and polish . (can't remember , its been too long ) this is how it looks after a 3000 km trip : I dindn't clean it up . there is no trace of oil seepage . The length of the tip is such that the felt ring exactly fills the gap between mainshaft and adjusting bolt . Felt ring is more to keep the tip lubricated than to keep oil in .