You would have to ensure there is only solid fibreglass where you plan to locate the in-hull transducer. The signal will not travel through wood, or any trapped air bubbles.
Fill a poly bag with water and put the transducer in it. Move it around the general area you are planning to mount it to ensure it works in that location. Mark the location and prepare a circle of a waterproof flexible sealant as a dam. Mix up some SLOW curing epoxy, and fill the area with it. Drop in the transducer and keep it in place as the epoxy cures. Slow cure epoxy when carefully mixed will not have any air bubbles.
I forgot to mention that you must carefully sand the area where you plan to locate the transducer. Here's a good link - https://support.norcrossmarine.com/h...-Illustrations