Just at this moment I'am doing the same thing.
Put the transducer on a piece of wood with two screws and fix that to the transom with an 'adhesive grip' the one you use to hold glued wood together.
Go out on the water and give it a try, now you can change the position untill you're satisfied.
It should be on starboard, at least 7,5 cm away from the outside of the prop, and 3 mm sticking out of the bottom. Be shure it's not in the way when trailering.