Trying to tie a secure loop/hitch in (s)trimmer line...

[KC]

Slick, too stiff to seat well, single, solid filament material.
Monofilament as closest material and even design match? 
Monofilament type knots?
.
Still think butane back splice or light tack weld after Standing Part and Primary Arc to later ropePart is good strategy too.

[Twine]

I suggest you tie the string to the pin with a stoppered clove hitch. Make the stopper a figure of 9 knot. A clove hitch with such a stopper is a very strong and secure gripping hitch and/or binder. Also, it's self-dressing, so you only need to pull hard on one part, the other is stopped by the stopper. I wish people would consider this knot more often, but I suppose it doesn't seem "pure" enough. Since it is made up of two knots, I guess most see it as just a simple improvement on a clove hitch, and nobody would ever attempt a clove hitch in this situation.

Anyway, pull hard enough on the standing (actually swinging) part, and the knot will grip very hard even on polished steel. Try it, and you may be surprised.