When I've been embedding my Prusa printer into the rack case I created simple filament holder with 25mm stainless steel tube used for a hangers in wardrobes (you'll definitely find something like that in your local DIY store). It worked but I found out that for MM printing some spacers between the spools will be useful. And because of large amount of variations of filament spools I have I decided to use some of the universal adapters for that purpose. But none of them were as universal as promised and finally I didn't like neither one of the solutions. So I decided to create the one presented here which works like a charm.

Covering most of the spools with various widths & holes I have, letting the spool roll easily on a 25mm tube which I use as a holder, doesn't take a lot of space (= not increasing spool width too much) and working like a spacer between the spools for MM printing.

And last but not least ... it looks great ;-)

Can be printed from material of your choice (PLA works just fine, PETG gives the best results), no supports required, 0,2mm resolution or higher is fine. Print time on Prusa i3 MK2/S is ~5 hours for both pieces together (bolt & nut) with 5% infill, 0,2mm layer and 100mm linear advance settings.

As it is nightmare to edit STLs there are also FreeCAD solids provided for a case you want to make some minor adjustments (increase the hole size etc.)

EDIT 10/15/2018: Based on several requests added spool rack holder (tube supports visible on photos) STLs (they are too simple to be published as single thing in my opinion).

EDIT 10/17/2018: Added _v2 files - both parts are 4mm shorter because of the Fillamentum spools I found, compatibily with other spools persists. Added _v2 of spool rack holder giving enough strenght and saving lots of filament on printing (just one file, for second side please use duplicating + mirroring feature of your slicer). Also tried printing it from PETG which looks to be the best / recommended choice.