Google just dropped 2D CAD drawings for its new Fitbit Air as an open invite for makers to design their own bands, so expect unofficial STLs to hit the usual model repositories fast. Just note: Long-term skin contact means sweat, friction, heat, and overnight wear, and very few 3D printing filaments are actually certified for that job. Look for materials, such as Siraya Tech’s Flex TPU 85A with ISO 10993 biocompatibility documentation.