This spool holding spring is made for spools of 50-55mm inner diameter.
The attached f3d (Fusion 3D, for Fusion 360) file is parametric and you can just edit the "md" in Parameters to the desired value (4-5mm above the actual inner diameter of the spool).
The "hd" is hole diameter, keep it 1-2mm above the shaft of the spool, the value here is 9mm.
This holds the spool by the pressure from the springiness of the shape, I'm making it with PLA but ABS would be a better option.
UPDATE: The spring is printed and in use, the spool moves freely and the springs don't go out of place due to the tabs.
UPDATE: This also acts as a dampener to the vibrations that come to the spool through the filament which is something you'd notice after you have dampeners (I have these https://www.thingiverse.com/thing:2127101) installed on the printer.
UPGRADED: I added flanges at the opposite side as well. Earlier it was slipping and the spool fell on it side preventing proper extrusion but with this update and lots of prints later it works great. The gap between flanges is 10mm which what most the spools have.
5% only (Grid/Rectilinear)
Print with 5% infill only, if you print with more then it will not have the spring action.