Metal detector test bed markers

I just got back into metal detectors after many years. We used to make "test beds" with common objects so you can learn to read them as they will actually sound in the ground. A test bed gets more accurate over time because the objects get oxidized and the metals "diffuse" into the surrounding soil emulating real hunt conditions.
I found over time that simple flags and such tended to become illegible or missing over time. But hey - now we have 3D printers and I decided to make something easy to spot that would remain legible and sit low so unlikely to get knocked off.
I made badges with common objects PT (pulltab) BC (bottle cap) penny with one copper and one zinc indicated with a 'z'. Dimes and quarters also with a silver and a 'c' for clad badge.

I used bamboo cheapo 5mm diameter chopsticks cut to about 20mm length to poke into the ground. I used an industrial grade epoxy to affix to the caps and the badges to the caps. I then used a marker (some I tried fingernail polis