It's OK to mount the plastic board to the transom with screws.

The idea is to have a sacrificial area that will take the holes and the new holes, and the holes that didn't work out instead of making Swiss cheese out of your transom.

When it gets too holy and its time for it to go to plastic heaven, just use the old one as a pattern and cut you a new one and mount in the same old holes you used to mount the old plastic board.