Windchill hi, yep we chose not to use sikaflex because as you say of its performance. Tried to take a seat out that had been glued in with the stuff and failed miserably. Ended up using an angle grinder.
just put a smooth layer of sealant in the hull and push the sounder into it. The important bit is to ensure there are no air bubbles which can affect the sounder.
Read another good tip last night of putting the sounder in a plastic bag full of water. Place this in the hull with the boat in the water and see what the picture is like. If it shows a good picture then go ahead and glue. If not just move it to another area.