I have bought some Really Useful Box(es), the smallest size, 0.07L. The AudioMoth fits perfectly! I will need to drill a small hole for the microphone (I'm not sure exactly where yet), and maybe some small holes at the bottom to let condensation out. The microphone hole I will cover with Scotch Tape which,hopefully, will keep the unit dry but allow external sound to be detected. There will be some attenuation, but I'm hoping that experimenting with the gain will overcome this.
I had to buy a lot of these little boxes and have plenty to spare if anybody in the UK or Euro region wants one - no charge. Email: email@example.com