Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4324718.

This is a slim protective sleeve designed to suit the Yubikey 5 or Yubikey 5 NFC (and perhaps other models). The main purpose of this is to colour code different keys, but it also offers some protection and, if you choose a bright colour, might make them slightly less easy to misplace.

This has 0.2mm clearance all round, and was a fairly snug fit on the two that I tried.

There are three different variants proved. The fully enclosed type (A) may be difficult to remove, whereas type (B) and type (C) have an opening at the rear to aid in removal.