A simple ring box I designed a while back, with two hinges. You can line the ring cutout and the upper lid with some cushioning if you want to spruce it up; and the outside of the box has been left intentionally blank so you can opt to laser engrave it after the fact. (Ignore the black 'grime' on the surface, that's just print bed adhesive from my test print. Shapeways print will come out clean). My test print was also semi translucent; you won't see the inner support structure in yours.
Ring box (closed): 9.9cm x 6.9cm x 1.9cm
Ring cutout: 2.8cm x 2.8cm x 0.9cm (+ 0.9cm above in hollow lid)
For something really special, try buying in 3D Printed steel, or maybe even silver or gold!