Nothing fancy here, just a remix of of my 32mm Tinkeriffic BB Bearing spindle. This one fits snugly in a 40mm spool.
This three-part solution (thirteen parts, if one counts the 4.5mm BB's) is designed to use a Tinkertoy rod as an axle and is a much, much smoother ride for your 40mm filament spools.
I found inserting BB's into the original 608 bearing to be frustrating so I split the bearing in two and left it to the end user to super glue the parts together.
The walls of the inner spool are by necessity quite thin; I recommend multiple shells or high infill for that part.