This printable ring includes little cut out sections to insert tritium vials - tiny glass tubes containing radioactive gas which glows like a glowstick for decades! They're perfectly safe, but do try not to smash them. A blob of clear nailpolish over the top secures them in place and adds some protection.
This ring requires about $20 worth of tritium vials, which can be bought from the-mad-nuclear-scientists on ebay. The dimensions of the vial should be 6mm x 1.5 mm.
The inner diameter of this ring is 17mm - you can find ring sizing charts to calculate what that's equivalent to. Scaling up will probably work okay - try to preserve the z height though. If you need to make the ring smaller, you'll need to fiddle around with the file in blender.
Partially inspired by the awesome designs of InnovoDesign on etsy.
I printed my rings on a Makerbot Replicator 2 using faberdashery plastic, spray painted black. The vials were secured in place with a blob of clear nailpolish. Good print settings on a makerbot-like machine are 3 shells, 0.1mm or lower layer height - no infill required. Print will take about 10 minutes on a makerbot.