Update Dec 1, 2018: Alternative design available: Parametric Auto-Rewind Spool Holder
Filament load/unload issues? MMU2 taking too much space? Not with this Auto-Rewind spool holder! It keeps filament on the spool during unloading, up to two full revolutions.
The secret is a spring element which keeps some amount of rotational tension on the spool hub while unwinding. The weight of the spool plus friction of the surfaces define the slipping point; nearly-empty spools will rewind less but that is no problem thanks to the high ridges of the (empty) spool.
Hub_Standard is suitable for standard 750g spools, Hub_Wide is used for the wider 1kg spools.
Two bearings are needed, type 608ZZ with outer diameter 22 mm, inner diameter 8 mm and thickness 7 mm. These are the dimensions of the MMU bearings as supplied by Prusa.
If you have different bearings or spools with a different centre diameter, you can customize your own hub with the Parametric Spool Hub.
If you are looking for an axle stand, have a look at thing:3205383 from Nicholas Bennett.
If you are using Slic3r, be sure to uncheck "Detect thin walls". The spring blade is 0.7 mm thick and (in case of a 0.4 mm nozzle) should be printed with two print traces that slightly overlap each other.
Update Nov 12, 2018: Added 3D printable axle, both standard (85 mm) and wide (115 mm) lengths. (Hubs with integrated bearings are not recommended due too much friction).
Update Nov 21, 2018: Added 3D printable axles with stop blocks that prevent rotation in spool holders with open-top axle slots.
Update Aug 4, 2019: Renamed to "Original Auto-Rewind Spool Holder"