Like Rick said, it's important not to have any air trapped under the transducer. One good way is to make a dam (mold) with a bead of RTV. Thin the epoxy with a few drops of alcohol (the alcohol will evaporate away, then the epoxy will cure). After pouring the thinned epoxy into the mold formed by the RTV, wait until all the air bubbles disappear, then carefully set the transducer into the epoxy and add weight to hold it in place until the epoxy cures. BTW use 30 minute or longer epoxy.