I can print NinjaFlex Cheetah TPU fine in the Prusa XL, but softer TPU filaments cannot be forced into the long and convoluted PTFE tubes.  This fixture snaps to the top side of the XL and allows filament to be fed more-or-less directly from the filament reel to the extruder/toolhead, through a minimal PTFE tube.  The pictures should make the setup and use clear.  A small magnet is used to keep the fixture in place.