Blast!!!
That's more or less the same thing I'm doing on my boat at the moment. Nice work from what I can see on the pictures. I mounted my "telescope" at the transom. Used 3/4 and 1/2 inch pipes and a jack plate to mount the transducer. For guidance I cut the 3/4 pipe for the length of the lift way and screwed a 6 mm screw in the 1/2 inch pipe.
I had a SAGA 20 (similar to yours) before I got my current boat. There I made a bracket to mount the ducer (EAGLE SUPRA PRO) at the drainage plug at the keel and drilled a hole in the plug to route the cable through. I had to cut the cable, but soldered it and had no problems after. Sealed the hole with epoxy and that lasted for over three years until I sold the boat without any leakage.
I'll post pictures of my "project" as soon as I finish it.
Nice craft manship dybvad417!!
EDIT: Looks even better with the new pictures.
SO THEREFORE HEREWITH DECIDED AND ANOUNCED:
As soon as I am the King of Germany, you'll become the
KING'S OFFICIAL TRANSDUCER MOUNTER!!!