Prusa i3 MK2/MK2S/MK3 Filament Guide with reces for cables / wires

Wrodz designed this filament guide: https://www.thingiverse.com/thing:2341182 - all credits to him.

I just added a few things to make it fit for my application:
- added a reces on the back side, since I have some wires running there for the lights
- added a very thin wall to the open loop: my first attempt printing this one failed, since the small piece at the open end got loose from the bed and ruined the print. With this version you can print is well and easily cut away this thin wall if