I'm not sure it this is of direct relevance to your circumstances and intentions.
I use the Garmin in one of its previous incarnations - the echomap 45dv. Unlike you, I specifically wanted something with chart plotting and SOG capability, but chose not to make use of the echo sounder functionality, preferring a NASA Clipper depth sounder. This was originally purchased for my previous Drascombe, but the same principle has been carried over to the BC20. The clipper display is mounted on the cabin bulkhead, and the transducer is 'bluetacked' to the hull on the starboard side, just inside an under-seat locker. I didn't like the idea of a transom-mounted transducer which would probably be ripped off at some inopportune moment, and the in-hull solution is out of the way, and works just fine.
Until I can think of a neater solution, the Garmin sits on its housing, attached to a wooden plinth which was designed to fit over the cabin entrance. When not in motion, the plinth is removed so as not to impede entry to and exit from the cabin. If I needed to put in the washboards, the plinth could just sit temporarily inside the cabin, or on the cockpit floor.
For the depth sounder, I like the Clipper display which uses large digits, easily viewed, whereas the equivalent on the echomap would be smaller and much less visible.
I use an 85ah battery which currently serves these two devices, plus a 12v charger for mobile devices. This is more than sufficient for a week's cruising.