I'm a "thru-the-hull" shooter.  

Many years ago, I took a short section of plastic down spout and trimmed it to the interior hull contour just behind the bulkhead in the bow of my canoe.  I then siliconed the edges to make a water-tight "cup."  I drop the transducer in the cup, pick up the small  section of synthetic sponge I cut down to fit the cup, dunk it in the water and add the water to the cup.  I then stuff the sponge in the cup to help anchor it in place ... its a bit over-sized so it provides a bit of friction so the transducer will stay in place even when the canoe is inverted.  I don't usually portage with it this way, but under most circumstances it will stay put for a moderate length portage.  I already have hull clips in place under each gunwale for mounting a rod tip protector tube (aka golf club sleeve), so I just run the cable thru the clips from the mounting thwart to the bow.  (I added one more clip on the bulkhead so the last mount is very close to the transducer, so even if it falls out of the cup, it doesn't fall below the gunwale line when the canoe is inverted.)

dd