Hi Jon,
In answer to your question, i used Sikaflex marine to stick the timber block to my Yamaha Fxsho, and yes it is holding great, wish i had mounted all my transducers this way on my boats, certainly saves boring holes into the transom. The Dragonfly seems good even in the sunlight haven't found any problems other than losing depth reading over 35 knots speed. I don't think the dragonfly transducer would be suitable for inside mounting as they heat up more and need to be in the water for cooling, plus they are long in size. I run mine on a small separate battery which seems to work well.
regards terry