Check out my SlavStruder, Its more beefy, less wobbly and a bit more durable than jstruder.
Also is already with tight extruder path to help with flex material
A simple mod that restricts a path that filament is going trough so you can print flex on a bowden setup.
Its only a filament guide part, you need to download a base from original source:
Capricorn xs or simmilas advised.
After printing (preferably with 5+ walls and 80% infill) you need to sand down a place where extruder gear will be located, it is made oversized on purpose so you can sand it down a bit at a time so that extruder gear does not rub on it but it still is as tight as possible.
I printed in pla but abs or other stronger material advisable.