This is actually a remix of some sample filament spool I found, but can't find the source anymore. (If you do know where it came from, PM me!)

One set includes a bearing hub where you can use any spool you want, it just needs adjusting in a slicer. The included hub is 42mm. 

The other is a one-piece spool that only requires the bearing. 

All that's required are two bearings and two screws.

In this particular case, I used an 8-32 2-inch screw and 2 bearings with a inner diameter close to the 4.1mm of the screw. I used heat shrink tubing to make up the difference in space.

The knob uses a 3mm screw, minimum 6mm, screwed from the inside.