I have skinny fingers and knobby knuckles so the ringsize that fits ends up being very loose and the SpO2, pulse, etc sensors have a hard time getting consistent reading.  This is a flexible spacer that can be inserted after putting the ring on witch holds it in place without being uncomfortable.

Print in Flex/TPU and I recommend the following changes to the default Prusa setting.  All of these are to ensure the outer perimeter is one continuous line with seams on the end so there are no uncomfortable bumps next to your skin:

Seam Position: Nearest  (keeps the seam on the end)

Fill Gaps:  unchecked (avoids extra fill which can result in perturbations of the outer perimeter)

Perimeter Generator: classic (keeps the outer perimeter in one continuous line)

My ring is 21mm inside diameter - can probably play with scaling to fit other sizes.

Update:  I added a new version ringSpacerVase which works well in vase mode:

• Layers and Perimeters 
  • Check “Spiral Vase”
  • Set Top and Bottom Solid Layers to 0
• Output Options
  • Check “Complete individual objects”
• Advanced
  • Change all (4) Extrusion widths to 0.8

Update2: I added a third options, ringSpacerVaseTall which has taller side walls.  I was losing about one per day with the prior design (they would fall out).  Have not lost any since using the tall version.  Isn't as discrete as the original but printed in clear TPU it doesn't stand out.