I didn't have a suitable o-ring for RedCowl's excellent Rival Pilot Swappable Inline Clip mod, but I did have a roll of TPU.  

Must be printed in a flexible filament, like TPU.

I.D. 18mm
O.D. 21mm
Height 1.4mm (print at .2mm layer height)

Note for pedants like myself: I don't think this is technically an o-ring, since the profile is square.