Filament Guide for Original Prusa i3, version 2.0

This is version 2 of my simple filament guide for the Original Prusa i3 MK1 or MK2. Like the first version, this guide lets you leave the filament in a box while printing, making filament changes easy. Improvements:

- It attaches to the back of the frame rather than the front, allowing the printer to use its full build height.
- It is tall to ensure the filament enters vertically even when the carriage is high.
- The attachment has a small lip to grip the frame.
- The guide has a tighter c