I've been working on and off on this for about a year, Was trying to work out how to do the case and the firmware. Had to track down a radioactive source for testing. more info can be found at the blog post below.
Blog post - https://facelesstech.wordpress.com/2023/08/19/diy-geiger-counter/
3D print files - https://www.printables.com/model/553527-geiger-counter
