I purchased my Ultimaker 3 Extended secondhand and found it was looping around the first filament reel when printing, so I decided to design a solution up.
This just sits underneath the rear and has a removable filament replaceable guide. This enables it to be fitted without removing the filament but also if it gets damaged you don't need to re-print the whole thing :-)
Printed without support at 0.15mm height in PLA at 10% infill.. works very well.
Please let me know how you get on and what you think.