This is a modified version of the base of the Printrbot Direct-Drive extruder (http://www.thingiverse.com/thing:281015). I've altered it to be more easily printable, and to reduce the gap between the drive gear and the base to make it more ninjaflex friendly.
Update 7/23/2014: I've tested this with PLA, ABS, and MatterHackers Soft PLA. It works great for all of them. I don't have any genuine NinjaFlex, so if you try it, let me know.
Update 7/23/2014: Minor updates to improve fit.
I suggest printing in ABS for maximum heat tolerance.
0.2mm layer height.
Custom support has been included, so print without supports.