I use a Black and Decker string trimmer for my yard. The auto-feed spools (AF-100) work really well at always keeping the right amount of string out. They swap in and out quickly when you run out of string. New spools are kind of expensive, so this jig lets you quickly re-spool on new string in a jiffy!
Video of the jig in use: https://www.youtube.com/watch?v=lg-0gDuBrSw
The print doesn't require any overhangs. The largest dimension is 4", so most printers ought to be able to tackle it. The winder requires a 1/4 inch bolt and nut to make it drill compatible.
UPDATE: Added a power winder that should be compatible with an M6 Bolt.
See more at my blog: http://kiltedcraftworks.com/2017/04/01/feb-mar-2017-3d-prints