I bought the 3D Jake rPLA filament (https://www.3djake.at/3djake/rpla-schwarz?sai=8009), and then realized that the cardboard spool didn't work with my spool holder ( https://www.thingiverse.com/thing:4368727).

That's why I designed an insert, which is simply inserted into the spool, and then you can use this filament on all spool holders.