If I always used the same canoe, I'd stick it somewhere out of the way with silicone caulk inside the canoe somewhere near me. As it is, I put the 'ducer in a Zip-Loc snack bag with a little water inside (and under). It's a little fuss every time but it works fine for me. The unit gets clamped to the thwart and the battery pack goes in a thwart bag along with the transducer and it's zip-loc for portages. Minimum cordage to deal with that way.

Any (exterior) mount I've ever experimented with made an annoying noise that I simply could not stand for very long. There were other downsides that have already been mentioned and you will need to avoid foam any core areas no matter what.

There was a removable caulk I once used to seal double hung windows temporally for winter 20 years ago. It's probably still around. I expect that would work.

My one regret with whatever is I'll tend to watch the screen more than I'd like to. It's good info to know but so is enjoying what's going on above the water so I do my best on that.