That's quite a project there, looks like it's coming along well.
Why not attach the ducer to the transom, cable too short or want it mounted mid-ship? How about making a long tapered fairing block that would house the ducer and attach to the keel, or leading and trailing firing blocks with the ducer sitting in the middle?
For fitting the ducer inside the keel I wonder it it would make a weak spot, or if building it up to compensate for the lost wood could create a hard spot in the keel? (Not a marine architect here, just spit-balling...)