The first thing I want to care about is the most critical part: the radioisotope and Geiger tube, plus shielding.

My choice of isotopes is pretty limited:

-U-238, contained in some old Fiestaware plates;

-Am-241, from smoke detectors;

-Th-232, from lantern mantles;

-Ra-226, from radium watch hands.

My choice is mainly dictated by how safe I want it to be: nothing in a powdered form, I want a solid source, that won't scatter easily if breached.

The Ra-226 is out of the question, as it is simply painted on zinc sulfide, the contamination risk is way too high.

Am-241 emits lots of alpha, but much less gamma, so not that great, since my GM tubes are only beta and gamma sensitive. Besides, there's a contamination risk should the source be released from its bracket.

I'm still hesitating on Th-232 or U-238: the lantern mantle has the advantage of being easy to wrap around the tube, but can scatter if burned (the cotton burns out, leaving a brittle ceramic structure that breaks easily and gets in the air, causing a serious contamination danger). On the other hand, the Fiestaware plate chip is solid and shouldn't scatter in case of a fire, but is much hotter (read radioactive) than the other samples I have; more shielding may be required.

As for the Geiger tube, I'll get started with a си-3бг (SI-3BG) beta and gamma sensor. It isn't very sensitive, that's why I need a source to get enough counts and use the thing. However, this tube is very small, so it should be easy to fit it in a small leaded enclosure.

I will use lead for shielding. As Th-232 and U-238 does not emit enough beta to create significant bremsstrahlung radiation, especially with the weak sources I use. Actually such shielding may look like overkill, but I want this thing to fit inside a pocket! I don't want to detect anything outside the casing.