Anet A8 simple cable guide (aka "filament" guide)

Update 11.06.2019:
To be frank, useless as a filament guide but super cool as a cable guide!

Update 23.05.2017: Since I switched to a Bowden I just discovered that this design works quite well as a cable guide, yay! (see picture)

Since my spool holder is behind the printer the guides I printed so far didn't work. In fact they made things even worse.
The problem is the steep angle in which the filament goes down to the extruder.
This design has solved this problem and lets the filament r