Prusa Filament Guide, with diagonal slot

As shown here: http://things.hands.com/thing/PrusaFilamentSpool/
this ensures that the filament on a spool mounted on my variant of the prusia spindle comes off the spindle cleanly.

The improvement compared with the version I based it on is that it uses a diagonal slot that does not need the additional flanges, or the suggested bolt to keep hold of the filament.