Yeah, I've done it with GoldTips, like gromage said. I've also used aquarium tubing, worked great for a little while, until the tubing started to shrink. Not good!

With weedeater line, you don't just stick little pieces of weedeater line inside the arrow like that. You use a hank long enough to go from the end of the insert all the way to the nock when it's bent in half. It won't move around when both the insert and nock are holding it in place, not to mention the light 'spring action' of the line itself, pressing against the walls of the shaft.

I've heard some folks are doing the same thing with soft rope or cord. I haven't tried it myself. I haven't had to figure out ways to weight up carbons since JeffB turned me on to CX Terminator Selects.