Customizable ring sizer.

thingiverse

Create your own jewelry ring sizer with various size options. Choose from three designs: individual rings, a single-piece tool with multiple rings, or a two-piece tool to prevent finger entanglement. Simply print STL files for odd sizes 3-12 and even sizes 4-12, then assemble using twist ties or rubber bands. This design is based on the math of Thingiverse's ring sizer (http://www.thingiverse.com/thing:7210), and has been tested and approved by three users who found it accurate with a slight possibility of being slightly loose.