The first photo shows the transducer and the supplied nut. The unit also comes with a block of black plastic (not pictured). This block can be cut to adjust transducer to level depending on the shape of your hull. I did not need it. Since I did not need the plastic block I made a large washer out of a white plastic cutting board. In the photo are the two hole saws used to make the washer, 2-1/8" and 1". The resulting washer is also pictured with the center plug. To cut the washer, first, make a shallow cut with the larger hole saw. This will create a center hole to index the smaller inner hole. Then cut the inner hole, then the outer hole. If the outer disk is cut first, securing it while cutting the inner hole would be difficult.
The second photo shows the starboard scupper on the T13A kayak. Note the notch to allow drainage past the transducer mounting. The underside of the scupper is molded as an inverted dome into which the transducer nestles keeping it safe within the profile of the hull.
The third photo shows the transducer mounted in the scupper. Top view.
The fourth photo shows the transducer mounted in the scupper. Underside view. It is a little hard to see since everything is yellow, but the scupper fits into a molded dome depression in the hull. The transducer has an egg shape and there is some controversy over which end should point forward, the wide or narrow end. The transducer is just a speaker and it does not matter for functioning. However, the scupper dome is shapped such that it seems to fit better (flatter) with the narrow end forward.