I would suggest a dense plastic mounting bracket glued to transom with the proper adhesive / sealant. Any Industrial plastic / adhesive store should be able to supply both. If you are coming by SE Calgary I can give you a piece of plastic and the adhesive (assuming it is still pliable and I can find it!!!).
The big advantage is that if you change sounders, mount transducer in wrong spot, damage transducer, tear it off on beaching etc. you can make changes without any holes in transom.
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