I sell these Splash Spools which are small spools of flexible filament. I originally started with 1.75mm filament but when I expanded my product line to include the 3mm size filament (after I bought a Lulzbot mini) I noticed the filament tends to flop off the spool when it gets near the end. This retainer holds the filament on the spool. It's designed to be loose so the spool can rotate inside it. I imagine this might work for larger spools as well but I my mini's platform is too small to make one.
Works better if printed in SemiFlex as NinjaFlex is a litte more tacky. For info on purchasing samples of 6 different types of flexible filament see flexyfilament.com