This is a thumb ring designed to help hold a book open when reading one-handed. It is not ornate. I kept it simple and functional. The thumb hole is 33mm in diameter. The thumb ring is not designed to fit snuggly around the thumb so the model is "one-size-fits-all". If the ring hole proves too small, you could scale the entire model up without loss of functionality. The pictured model was printed in ABS in medium quality mode with a 30% fill. I would imagine PLA and other printing parameters would have no real effect on the basic functioning of the ring. The ring can be worn one of two ways; with the ring side toward the book, or away from the book (as pictured). Both work well and your choice should be a matter of comfort. I have included both stl and obj files. I should say that the design is inspired by a recent Instructable (http://www.instructables.com/id/How-make-a-reader-ring-with-Sugru/). Enjoy your reading.

designed in TinkerCad
sliced in Cura
printed in ABS
layer height 0.2mm
wall thickness 0.8mm
fill density 30%
temperature 260
print speed 50mm/s