Delrin 4 mm wide D bearing Blank
$5.14
These are designed to convert d bearing yo-yos into fixed axles. Tested myself in the YYBC WorkHorse, Ti Respawn, and Dessel Oasis. May not fit in all d bearing throws, depends on post height of the design.
Word of warning, Delrin is softer than Ultem and these can be over tightened if you are super aggressive.