I have a SR 18.5 and I am able to shoot thru the hull without issue.  What I do is, I have a piece of closed cell packing foam about 4" x 5" x 1" that I traced the transducer outline on.  Then I cut a hole slightly smaller than the outline so I can push my transducer into it and it is firmly held in place and upright.  I then silicone the foam in place and it allows me to easily remove or install the transducer with just a splash of water to maintain the signal.  I have found the silicone sticks better to the canoe than the foam, so I drilled holes in each corner of the foam and injected silicone to make a soft rivet to hold it in place.   The silicone does come off the canoe with a gentle scrape from a plastic putty knife, leaving no marks.