So I originally only made this an an upgrade for my own printer, but by popular demand I have decided to upload it for every one to share.
The idea is simple, I use allot of Rigid Ink 300g spools that have a 50mm inner diameter, but my spool holder is 30mm so I designed this adaptor to save changing the spool holder for each spool.
It comes as one half and would need to be printed twice of course but both half's are the same so that wont be an issue, you will also need 4 4mm x 1mm rare earth magnets that fit pretty snug when printed correctly but may still need a spot of glue to hold them in (or just simply glue both half's together permanently, if you have the space to still slip it onto your holder)
As this is a functional part not for show I only printed mine light and on a quick resolution, but these details are up to you.
But I would keep the walls 1mm as you will need to fit the magnets.