What a nice little high-voltage supply you have built for your GM tube!
What voltage is required? I ask because I had to replace my fly-zapper recently. The blue-through-UV LEDs had died but the 4kV power supply still works fine. Perhaps it could be re-purposed.
I am also tempted to build a spark chamber. Lots of fine wires across a cube about 1m in each dimension with each wire being just higher than the breakdown voltage of undisturbed air. Such a beast shows muon tracks very nicely. Elfin safety dictates that the whole be encased in something like Perspex sheets to keep sensitive bodily parts away from the zappy things.