for anybody looking into a small contained and power independent solution for radiation monitoring I put up my own project with some build instructions
The code/sketch for the micro controller could be changed easily for other boards as I used the cheap ESP8266. The casing is inside the github repository and should fit most designs. The small area with
cutouts is meant as a protection for the mica end of the Geiger tube.
Currently just soldering skills are needed for the custom connectors. A possible further upgrade would be a PCB design without the clutter on the used Sparkfun board (the unused micro controller and the LED take up some mA)
and without any other power hungry but dormant ICs (the Serial/USB bridge on the ESP8266 for example)
but currently I have no ambitions to make this a fully fledged product. Any pull requests welcome.