Getting a transducer in the correct location to get best results is important. And, of course that spot should be located before any drilling is done. The transom on my aluminum boats are perfectly flat so I just attach my transducer to the end of a short (21 inch) length if 1 x 4. Then use what I call a rachet clamp found and any Lowe's or Menards to clamp it to the transom. It is easy to adjust and can be changed while the boat is in the water. Keep in mind there are three things involved with getting the results you want. An area with clean water (no bubbles) coming under the transducer. A transducer set at the right height. Too low will catch debris and have a better chance of being knocked loose. Too high may not keep good contact with the water. And finally the direction the transducer is pointed. The water is level but your boat does not necessarily set level in the water. Adjust the transducer accordingly.
Remember holes are pretty much permanent so,,,,,,,, Measure TWICE and think THREE times before drilling.