I've made an initial attemp to design a 3D printed case for an audiomoth (with 3 x AAA battery pack) and have made it available on the thingiverse (https://www.thingiverse.com/thing:3292311). It includes a 3D printed rubber seal for use with a rubberised filament. The case lid needs to be printed without any in-fill in order to be strong enough. There is a strengthened section adjacent to the microphone hole that will need to be drilled and ideally covered with waterproof speaker cloth.
In the next version I'm intending to include some structures to allow easy fixing or strapping to flat surfaces and posts. I don't know whether it would benefit form for example a conical aperture rather than a small hole to guide sound to the microphone.
I was hoping that together we might be able to come up with a design that will suit most of our requirements.