This is designed for everyone who subscribes to one of the multiple monthly sample filament deliveries, or if you but a multi pack sample like the one offered by Polyalchemy.
These are designed to be easy to reuse and swap the sample you are currently using. There are opening in the middle of the spool to hold onto the end of the sample so it doesn't unravel and the slot openings in the side are designed to go below and above the sample this way you can easily place the twist tie back onto the sample while it is still neatly in the spool.
The custom part of the spool is the design of the sides.
You get to pick how it looks, I made it specifically to take advantage of the modifiers in prusaslicer
You first load the part you want (big or small) and the load the provided modifier stl
Or just load the emf file provided.
The you can make the top and bottom layers of the main part 0 to expose the infill design.
Prusa released an excellent video on the process