Thanks.  Here is a picture of my previous setup of foam block(s). The white one is what this replaces.  The black one was for a HDI transducer with traditional and downscan, though I found that downscan frequencies do not transmit through a hull like traditional sonar.
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I probably could have solved my intermittent dislodging issues by simply using a taller piece of foam, such as the black one, which would cradle the front fin of the transducer, keeping it upright.  The other issue I had with the foam is that I found that silicone does not adhere well to closed cell foam and through the flex produced by inserting and removing the transducer multiple times over the years, I lost the water tight seal this vessel was meant to provide.  It still worked though as it's position under the stern seat is where water naturally collects, so there is typically a small puddle there anyway.  The four holes in the foam around the transducer cutout were filled with silicone and that is what actually held the foam to the canoe long term.   I found the foam peeled away easily from the silicone, but it took a bit of effort to remove the silicone from the canoe. 

The transducer well has a groove designed into the base to act as a gasket and add surface area for silicone adhesion.  So far it feels really solid and since there is no flex or movement, I expect it hold up and perform well.