Duct seal, aka electrical putty is the only way to do this. Get a golf ball size of this stuff, about $3.00 for a lifetime supply at HD, and stick the transducer to the hull with it. Mash it down as hard as you can. Go fishing. You will not be able to pull it straight off. You will have to pry it up from one end which takes some effort but not much. I have done this with kayaks for years.