Simple Filament Guide for Wanhao Duplicator i3 Plus

I switched to using [this](https://www.thingiverse.com/thing:508896) spool holder for the i3 Plus, but found the filament was rubbing against the threaded rods from the [z-axis brace](https://www.thingiverse.com/thing:1653631), so I designed a simple filament guide to clip onto the top of the frame and guide the filament over the rod.