This thread reminds me of Encouraging a sense of scale in photos.
Hands are available in a pinch, and coins are helpful, but non-standard, and often lacking after buying all the necessary camera gear for iNatting…
What about something as widespread as a glass bottle cap. Are they standard across most of the world?
A big plus is that if a specimen is small enough, the cap acts like a dish/container to keep it from crawling away. Also, local culture can be shared depending on the bottlecap design.
New edit, on the subject of hands for scale, I’m not sure if anybody thought of this option yet. Possibly the next innovation in natural photography, right after digital photography.