Signet ring with cavity to hold a tiny RFID tag. The embossed image is a fanart derivative of famous Hadouken, symbol of our RFID Hacking Discord instance. It can be easily changed by changing the PNG.

The RFID tag I'm using is self-made from an ISO card, cf https://twitter.com/doegox/status/1330515209803476995 thread for explanations and pics.

The OpenSCAD source file should make other adaptations easy.