I think a lot of the Bass boat guys mount their transducers to
the jackplate.
I don't use a jack plate, but you might consider this:
The jackplate depth could change depending on the trim/depth of your motor.
The optimum jackplate depth for your motor, may not be the optimum
jackplate depth for your transducer.
Also, you want the transducer it at least 15 inches away from your prop.
Hopefully somebody that has mounted it this way will post.