[BigLew]

Warthog,
That's ULTRASOUND coupling gel, not glue or adhesive of any type. Absent using a surfactant of some sort on the ducer to minimize the formation of bubbles, a coating of the ULTRASOUND gel on the face of the ducer should minimize the potential of bubble formation as it is placed into the mounting collar, or box as you call it.

What you are trying to do is to form an ultrasound conducive coupling between the face of the ducer and the water. Should there be any voids in the fiberglass or column of coupling medium (air bubbles in the gel or antifreeze), that will have a negative effect on the transmission/reception of the ultarsound signals being transmitted and received by the ducer.

I'm just trying to offer a sound bit of information.