Finally finished installing my new sounder today. As you can see, I've made a fibreglass housing to stop water ingress into the plugs on the back of the unit. The plugs do have an O ring seal on them, but I think this will give me a bit more peace of mind. The front of the housing also has a seal inside where the unit is recessed. This was achieved by running a bead of sikaflex around the inner lip, and allowing it to dry before screwing the unit home.
The ram mount has been incorporated into the fibreglass, as have the nuts that the mounting screws for the unit screw into.
The rest is pretty standard stuff.