Some of the companies state in their product description that the in hull transducers work as good if not better than the thu-hull types. Toilet bowl wax sounds good. I had similar results with "shoe goo." Sometimes you have to play around with location due to trapped air in the laminate.
Originally Posted by BarryL
If your boat has a solid fiberglass hull, installing the transducer should take about 1 minute. You can use a 'puck' type transducer instead of a through hull. You don't need to epoxy the transducer or mount it in a tube filled with oil. All you need is some toilet bowl wax (about $1.00 from Home Depot, etc.). Press a wad of wax onto the hull where you want to place the transducer, then press the transducer into the wax.
I did this about a year ago. I originally planned on using the wax to find a good location, and then epoxying it in. However, since it has worked great, I have no reason to glue it down.