OK, the tensioner knob is not really necessary - it is more an esthetically aspect for me (look at image 2 and you know what I mean). But I promise you: you will never need a tool to adjust the tension on the x-axis anymore.
See also my y-axis knob here and my bed leveling knobs here
The Customizer automatically creates different shapes, it took the "m6_tensioner_knob_rounded" one.
If you got problems getting the knob in place, simply heat up the nuts a bit (about 60-80 °C / 140-175 °F for PLA) and it easily slips in place.
I have added the original SCAD file with modified start values for easier creating similar knobs.
Customized version of http://www.thingiverse.com/thing:713300
Created with Customizer! http://www.thingiverse.com/apps/customizer/run?thing_id=714498
I print mine with 0.25mm layer and 20% infill.
Using the Customizer with following options:
knurled_strong = 4;
hex_nut_width = 10;
knurled_height = 100;
hex_nut_thickness = 10.5;
bottom_thickness = 0;
screw_hole_diameter = 0;
diameter_2 = 100;
height = 11;
V1.2: flip very long knobs (miniature hex handle) and new hex shape
V1.5: new support of tetragon up to octagon nuts
new wing shape
error in calulation of nuts fixed (relevant on bigger ones)