regarding enclosures you get started with the battery tube + endcaps. Gives us approx. 1 1/8” inside diameter. Without the batteries in, the tube would flex or collapse under pressure. You could go with a short tube or add plugs (or actual batteries for power) for structural support.
Another simple approach would be to sandwich together (with maybe 5 to 6 brass bolts) a couple of acrylic disks (like the endcaps) with a center section with an o-ring sealed thicker disk (would 3/8” be enough to fit electronics?), that has it’s center cut out (smaller diameter circle) like a donut. The cut out section would be large enough for the electronics and batteries. Wires would be run through a potted hole and the transducer would be mounted at the center outside surface.
A friend of mine built a simple hydrophone using a piezo disk this way. https://ccrma.stanford.edu/~sleitman/Hydrophone/
I have the unit if you want to give it a try (use from the surface only with amp - like a radio-shack mono amp)
I would like to do an enhanced version of the hydrophone as an add on to the ROV for listening to ocean/marine animal sounds. Would have to sample and stream audio to the surface.
With FET pre-amp like this: