Essentially, the design is a cylinder with a longitudinal slot in it. Inside the cylinder a hair curler/roller with nylon bristles fits and can be rotated using a knob which acts as a cap to the cylinder as well. It works thus:

1. Push bit of the spare line into the slot.
2. Rotate the knob which…
3. Turns the hair roller which entangles the line and…
4. Pulls it into the cylinder.

It actually works a treat!

There were a couple of major design issues:

* Working out how to 'attach' the knob firmly to the roller and then hold the assembly within the cylinder whilst allowing the whole thing to rotate. This was done by fashioning a spigot on the knob which firmly fitted within the roller and fashioning an annular lip on the knob which clicked into an annular recess within the cylinder. This required bit of trial-and-error to get the dimensions correct.

* Making the knob. 3D printing doesn't like 'overhangs' so the design is in 2 parts, one a hemispherical cap with rounded recesses and an 'armature' which contains the spigot and lip mentioned previously. The 2 bits are bolted together with an axial bolt and self-locking nut.

I think the image to the left (or above on smaller screens) shows most of this reasonably well.

I actually printed mine in 2 different colours and it looks pretty neat!