The epoxy cast turned out great. A two-and-a-quarter inch hole turned into a two-inch hole. Just a few rough edges to sand down.
The new through hull looks good. Lots of LifeSeal (BoatSeal) polyurethane/silicon hybrid caulking around the through hull and up over the threads.
With the transducer in place it's ready for a coat of antifouling. Is it water tight? We'll soon find out.
The transducer is an interesting device; the small paddle wheel on the outside of the transducer spins and interacts with a magnetic sensor on the inside. The sensor is good enough to pick up the frequency of the wheel's rotation through the plastic plug and send it to a instrument panel for conversion into a digital readout. As a bonus there's a temperature reading to test the water on those days when you really feel like going for a swim.