If you want the sonar to work correctly for shoot-thru, the transducer and the hull should be one piece. That takes a solid connection with EPOXY.
ANY flexible material will absorb some of the sonar pulse strength so it cannot work full strength.
You have to remember the transducer transmits sound and the transducer face and hull have to vibrate at the same rate. Then the returning sonar echos have to be detected by the hull and to the transducer face at the same vibration rate.
The next best shoot-thru installation is a pool of liquid with the transducer inside so the liquid forms the solid bond to the hull. That liquid can be water, oil, anti-freeze, etc since it does not compress like putty or duct seal.
ANYTHING less is just that----LESS
Additionally the hull material has to be of a solid nature, without any aeration that would keep it floating if submerged.
Externally mounted like flyfisher suggested is the best way if you do not use epoxy or the pool method.