I have had some previous success with 3M5200 'ing a block of Corion/cutting board/star?? solid material to the exterior of the transom. I then drilled and mounted the T'ducer into it. No holes in the transom.
I made the block oversize so I could change out the mounting or put on another one if the original unit broke.
Didn't come off for years...and then I sold the boat. regards, Rich
p.s. Kitchen counter shops often have cutoff pieces or sink cutouts that are free or at little cost.