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I use aluminum, about 1/2 or 5/8, cut into a square. I'll drill and tap that to match the screw pattern of the transducer bracket. Now, I can use SS screws and just bolt the transducer to the aluminum plate.
To attach the plate to the boat, I'll clean the surface of the transon well, rough sand the back of the plate and cement it to the transom with Gorilla Glue. Holds well, no drill holes in the glass. Gorilla Glue sets by contact with water. It doesn't get any better than this!
Good Luck,
Grady