I really liked the self-centering filament holder by 3D Sorcerer, so I remixed it to fit on my RepRack. This spool holder works well for my smaller filament rolls, and doesn't need to be adjusted in size.

I have currently only supports the standard-sized rollers, but I believe this could support larger rollers by just replacing the long hex rods with longer ones.

You will need to print:

2x Body Half
2x Long Hex Rod
4x Roller
2x Roller Hex Rod

You can also alternatively print the rollers as one piece from this remix: https://www.printables.com/model/557004-self-centering-filament-spool-holder-roller-as-one

Additional hardware:

4x 608 Skateboard Bearings
2x M3x12mm screws (optional but nice to have)

The two long hex rods keep everything rigid and prevent the body halves from shifting laterally. They are friction fit into the body.

You can use a couple of M3x12mm screws to secure the spool holder to the RepRack. These screws will keep it from moving on the rack.