I know this is not recommended but I was able to install my transducer with 3M Double sided tape. This is the same tape used to put body pannels on most cars and Trucks.
I chose this method because I could not determine the best location for my transducer and I wanted to test it before I started drilling holes. The tape held so well I have left if on for three months. And if I ever decide to remove it, you can cut the foam tape with some braided line in no time at all.
This may not work on a highspeed boat, but on my aluminum flatbottom and a 20 hp mercury it holds tight!