Transducer suspended in vegie oil. square to the bottom
The type of transducer you are speaking of is called an "in-hull" as opposed to a "thru-hull". The in-hull transducers can be purchased for around $70 at West Marine for one. They come with a suitable mount ready to epoxy/glue to the hull.
If you need a transducer that is probably the best way to go. If you already have a transducer, and just want to mount it, you can make a suitable mount/container out of PVC pipe.
One thing to be aware of is if your hull is cored in the area you wish to mount the transducer, you must cut out the core material so the transducer is "looking" through a solid laminate material.
Mineral oil is the usual liquid used to couple the transducer signals to the hull.