My transducer if fitted in the sump - no holes were drilled through the hull that I can see (I bought the boat second-hand so did not see the original installation). The unit is fitted on the lateral center line of the boat at the lowest part of the sump just in front of the transom drain plug. (Be sure that you can remove this plug after fitting.) The only prob with this position is that I have dificulty in getting the hull/deck drain plug out.
The previous owner did tell me that it was necessary to keep a small amount of water in the bottom of the sump but I'm not too sure why - anyway there is always some in there anyway.
Araldite is a two-part adhesive that we have over here and will stick almost anything to anything else for good
I think you have a similar product over there called JB Weld. However, I think Wavelength's idea of silicone would be better for easier removal if needed.