The ring from a coffee can lid should work fine. Forget the tape and just use a caulk of some sort (my favorite is "Amazing Goop", found at most hardware stores.) The only reason we originally used tape is that while on the way to our fishing destination, we decided to stop an purchase the fish-finder. Tape is what we had with us.
As long as you don't care about water temperature, shooting the signal right through the hull works fine.
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