I have not used that epoxy before but know that the longer the cure time the better off you will be. Just make sure you are using it in the temperature range specified by the epoxy manufacturer. If you use it in at a cooler temperature the cure times will typically be longer and shorter if it is hotter.
I would use a plastic bag filled with dirt/sand/pea gravel to hold the transducer in place both while testing and while waiting for the epoxy to cure.