This is a modification of the hub by 3d-workshop to fit spool refills that are just a little too wide for the original spool. You can get the sides, reload key, and the original hub from here: https://www.thingiverse.com/thing:2823667
Update: Added a hub that is slightly smaller for Wanhao Duplicators or Monoprice Maker Select since the first one was too big to fit on the stock spool holder.
Pro tip: Use the smallest hub possible. If it is too big, your filament won't be tight and it will get messy and hard to print. Only use this mod if the original hub is too small.