That hobie solution seems to be the best way to go, the problem with all other methods when you mount it inside the hull is that they all need to be redone at some point as airbubbles always end up developing.
the best inside the hull solution I have seen is the humminbird set up (but it only works with their ducers) here it is:
http://www.amazon.com/Humminbird-MHX.../dp/B001I7SMJK
I used this for two seasons, but because a tiny bit of water gets into the hull it slowly wore down the glue that holds the little frame thing on and the grease stuff that the ducer sits in. So I just re did it with super glue and reel grease and it works great.
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