Just buy a bracket from Lorance for your model transducer. probably less than $10 and it should come with the mounting hardware too.
Or if the floor is one layer plastic you can epoxy the transducer to the floor inside the boat. Works just as good. Rough up the area a little with some sandpaper. Take some clay and roll it out like a big worm. Press that around the transducer, remove the transducer and fill the little clay pond with liquid epoxy that sticks to plastic. Press the transducer in real slow so you don't get any air bubbles under it. When dry, remove the clay.
