Just did the same
We just put an auxiliary mount on our Cal 29 for a 15HP Johnson. It did the job to get us out of a pinch when the main Atomic 4 decided to quit (as it has been known to do). Mounted a bracket from a Catalina 27 in the center of the transom with a backing plate. It has been a comforting addition, but as mentioned in a previous post, cavitation in choppy seas is definitely a concern. Best of luck.