This is a 32mm (radius) spool for offcuts of wire to keep your electronics box tidy. They hold about 16m of 'jumper' wire.
The cover is optional but keeps the wires from springing out.
The cover needs a brim except for the stickiest of print beds due to the low surface area touching the bed.
the two sides of the spool snap together with force and are unlikely to come apart after assembly, so I would suggest threading the end of the wire on to the wire holder in the spool and through the hole in the cover before fully assembling, you can then fully assemble - if you are not using the cover then this doesn't matter.
I've included the sketch-up files if anyone wants to modify, correct, improve the design.
A brim might be needed for the cover