This is a spool holder that I modelled to replace the spool holders at the back of the J1s. It was modelled in Blender using the Add Mesh: BoltFactory add-on to make the threads.
The spools were printed in Sakata 870 PLA. The temperature was 230C, with a bed temperature of 60C. It was sliced in Luban and printed at 50 outer, 50 inner and 100 infill. Breakaway supports were used on the overhang surfaces of the shaft mount. This was my first use of the breakaway filament, that came with the J1s, and it worked really well. No support was needed for either the nut or the bolt threads.
There are two 8mmx22mmx7mm bearings on either end of the bolt. The shaft friction-fits into the bearings, and a cap is mounted on the end to keep the bolt from falling off. There is a 4Mx20 hex screw placed inside the bottom of the shaft to give it strength. The cap is screwed on with a 3Mx8 hex screw.
To prevent unspooling, a small piece of open cell foam is placed between the shaft mount and the bolt back. This applies a small amount of resistance to turning.
To use it, unscrew the nut a couple of turns, remove the old spool, put on a new spool, and screw nut back on.