Connecting the motor to the slow motion control

This is that TG-SP II motorized guiding project.

Last night I really try to place the motors around the slow motion control knobs to see different possibilities. The shaft under the knob is 6mm in diameter, while my motors are having 3mm shaft, and I find difficulties in locating 6mm-3mm inline shaft coupling, all I could find are those 6mm to 5mm or maybe 6mm to 4mm (unsure if I saw it?), but I guess maybe that is enough, or in the worst case, I just find other motors.

There're two potential issues by using inline shaft coupling. First of all, the motor will extend all the way from the shaft, while the motors are pretty small ones, I don't know if it will make the transportation of the mount harder, since we got something sticked out which can be bumped, not exactly a good idea. Secondly, I found the shaft hard to turn, at least it cannot be turned with my fingers. The motors are pretty powerful, but I have to try before I can really tell that the motors are powerful enough. I suppose that they can do, since some motorized focuser work like this, i.e. directly drive the shaft.

There're two other possibilities, they're similar. First is to use two sets of gears, the advantage is that, the gears can be bought to match the size of the shafts, and the motors can be placed side by side with the shaft under the knobs, thus making them the same as with the knobs, i.e. nothing protruded outside like that of those inline couplings. Secondly, I could choose suitable gear to make the driving torque stronger, and this will ensure that the motor can turn the shaft effectively. Another possibility is to use timing pulley and timing belt, for that to work, I believe that the timing belt can be used to drive the existing slow motion control knobs directly, and it works similarly with the gear as well, but problem is, I probably need to derive an engagement mechanism for the belt, i.e. to make it tight enough to drive the slow motion control knobs. That part could be hard to DIY.

It terms of DIY ease, the inline coupling way is the best, but it's not the best design overall. Timing pulley is the best design I'd say, but it's hardest in terms of DIY.