I made a new extruder top for the Bowden extruder that came on my Tevo Tarantula so it will feed flexible filament. I have not tried Ninjaflex, but it works great with Makerflex from Makergeeks. As you can see in the photo, it is basically a copy of the original with tighter tolerances so the filament can not escape out the sides instead of feeding into the tube. The hole for the tube fitting is for a 6mm thread fitting. I used a different fitting, but the original one should fit.
Edit: Someone requested the bottom half, so here it is. It is made for 4mm bolts on the roller and spring holder, and the screw that goes into the stepper motor is 3mm.
The roller is 13mm in diameter. It is the one that came with the Tevo.
Have fun and happy printing!
0.4 Nozzle, 0.2 layer height
You may need to run a drill bit through it after printing to remove any burrs on the inside of the throat.