A keychain that holds a full size SD card. The key ring acts as a stopper to keep the card from falling out.
The 3x stl is there to make sure the model can be printed upright, as sometimes tall and skinny models have trouble staying attached to the print platform.
EDIT 2017-07-28: I just uploaded a v2 of the model, after the keyring broke off my own card holder. The new version has more material around the hole to make it stronger.