Hello,
What a nice little high-voltage supply you have built for your GM tube! A few years ago I built a little muon detector to the MIT design (which seems to have morphed into the Cosmic Watch detector). It uses a silicon photomultiplier and a scintillator block and is controlled by an Arduino Nano. I have attached a picture, just to show off. I always get counts of around 17.5 per minute, a bit less than you do, and not much variation during the day.
