This guide clips onto the top of a Prusa i3 and prevents the filament from tugging against the sides of the spool when the X axis is near an extreme end and/or the spool is nearly empty.
Filament can be fitted into the guide even when it is already loaded into the extruder. Simply turn the inner guide so the slots lines up, insert the filament, then turn the inner guide to prevent the filament from escaping.
With a small piece of sponge or foam can be wrapped around the filament to further support it and a drop of oil can be added to smooth the filament's path through the extruder.
This guide can be used with any spool holder as it clips onto the Prusa independently but is best used with my Universal Spool Holder https://www.thingiverse.com/thing:2504871. It is especially useful for guiding loose filament hung on my Loose Filament Holder https://www.thingiverse.com/thing:2532195
You'll need an M3 10mm (or longer) bolt.