This is my personal approach for a filament guide fitting the Anet A8 printer.
All the others are just a support to attach a circle to the frame. This one has a pulley included for a smooth filament move from the back of the printer as I cringe everytime the filament scratches the frame.
Only use supports for the actual piece, not the pulley or axis, or they wont be smooth enough!
The guide doesnt have a cut to insert and extract the filament because from time to time it slips out and you need to remove the filament from the extruder anyway to change it.
Pulley and axis are included for a print&play experience, but maybe some people prefer to put an actual pulley with bearings, etc. The included ones turn pretty smooth anyway, just make sure you fix the axis with a couple rubbers or something (like the small ones that come with the A8) as its a bit smaller than the hole (3.5mm vs 4mm).