I also have a Zodiac Cadet 310s. Mounting the transducer in the ways mentioned above didn't work for me. I found another solution which might be helpful for others who end up at this forum. I used PVC plumbing pipes to make a construction whereby the transducer hangs in the water at the side of the boat. The diameter of the pipe was 32 mm. Inside the tubes that form the foot of the construction (inside the boat) and inside the tube that runs down into the water, I slid a lead pipe and held that in place with little screws. It all works perfect. When I want to go fast, I can even lift it out of the water. 
