I've used a few different spool rings (I now just use electrical tape), and spool adapters. This method (with the bolt) seems to help with cardboard (even plastic) spools that are warped or otherwise damaged. Also prevents the rare occasion of the two sides separating. The bolts I wound up using were very inexpensive (seems to have been a price mistake that has since been corrected. Now $24.86, was $1.28 for the 25 pack) (I was previously using hex head bolts, but they weren't available locally at 2 1/2").

Parts:

• Deckmate ceramic coated black carriage bolts 5/16"-18, 2 1/2" length.
  • https://www.homedepot.com/p/DECKMATE-5-16-in-18-x-2-1-2-in-Black-Deck-Exterior-Carriage-Bolt-25-Pack-867750/321056975
• Everbilt wing nuts 5/16"-18
  • https://www.homedepot.com/p/Everbilt-5-16-in-18-Stainless-Steel-Wing-Nut-15-Pack-807532/204281876

Print both halves. The wing nut side has supports turned on, the square side does not.

Feed the bolt through the square side until it rests inside the square. Insert the spool. Attach the wing nut side to the spool threading the bolt through the hole. Attached the wing nut, and tighten finger-tight.