The spring design requires the filament to be somewhat flexible and elastic. Design files are linked below for anyone who wants to adjust the spring thickness / tolerances. PolyMax isn't quite elastic enough for this spring, but Colorfabb PLA/PHA (which is more flexible than normal PLA) seems to work well. A new version is available, this one should works better with the more brittle plastics.

Printer:

-- Eustathios

Design:

-- OnShape

Filaments: (full list can be found on thrinter.com)

-- Colorfabb Blue Grey PLA/PHA

-- Polymaker PolyMax White

(filament donated by PrintedSolid)