I managed to get a 50/30mm rubber adapter from local hardware store, and tried if that fits. The picture below shows the 8×50 finder scope after unscrewing the 2 back pieces out from it. Under these 2 pieces one can see also the 50/30 rubber adapter.
Luckily this worked! The connection is very tight, and one have to be carefull when using the force.It is better to connect first the rubber adapter and then the CCD adapter, and CCD as the last one. The final focus adjustment can be done with the front (objective) lens, because there is a separate locking ring (left in the photo).
This setup should work with moderate focal lengths. I have telescopes with 500, 900, 1200 and with 1800mm focal lengths and I think this will work at least with the first 3 ones.
I have not use autoguiding before, so let’s see how this will turn out… (There are good software alternatives, eg. Open PHD guiding .)