I would plan on going with a drill less whatever that it.
First thing I did when I bought my boat was to fill the old transducer holes with epoxy and mount a transducer mounting board.
I have since gone to a thru hull for my primary, but use the plate to mount side the scan transducers and stand alone paddle wheel spped
For easy mounting, addition and replacement of transducersMarine grade polymer will not warp, corrode or be affected by UV raysMounts easily with two screws
www.iboats.com
Used the drill, fill (epoxy) and redrill method to mount the board to the transom.