I have a big floor fan that constantly tipped toward the ceiling because the adjustment nuts on it were loose. It came with knobs, so the nuts could be tightened by hand, but those knobs fell off and got lost. So now, new knobs!
Hopefully I correctly understand what "parametric" means. The .scad file contains a single module call whose parameters can be adjusted to change the size and shape of the knob. If I misunderstood the point there, go easy on me, as this is my first Thing! Hope you like it!
Inspired by http://www.thingiverse.com/thing:11326
Setting nut size - I set my knobs to 13 mm and they fit the 1/2" (12.7 mm) nuts very snugly (which is good!). It probably depends on your nozzle width; mine is .4 mm, but YMMV.