When I first had to deal with that issue, it was on a 2.5 hp 5 mph Sea Eagle rig so I just got a big hose clamp, attached the transducer to a big Arizona Iced Tea bottle, and pulled it a couple feet behind the boat. It worked surprisingly well, and led to an expansion into full portability by using a 8 x AA battery box from Radio Shack to get 12v DC. Used it while shore fishing with great success and allowed for resale for much more than I'd dreamed possible (non side-imaging HB).
Now, I'll be using a 10' inflatable with 8 hp as my second boat/RV rig, and the tea bottle probably ain't gonna cut it. Plus, it's kind of an insult to my beloved 997C. The best idea I can come up with so far is a modified quick-remove bracket with a screw clamp.
I'm in the middle of building a zombie apocalypse 4x4 expedition vehicle, so I've not yet made it to the point of sinking my teeth into the transducer issue, but high on my list for dealing with it is a trip to the iboats forums, which contains a subforum for inflatables. Lots of people who know their stuff there, and I'd expect a good solution is rattling around in one of their melons.
Good luck....to both of us. Sigh......it just dawned on me that I gotta get another GPS puck, too. And another knob for the RAM mount. It never ends. So goes having two boats, but only one "Golden Child" (our name for the 997C, which, by the way, I got for just $650 over 2 years ago).