Here's a link to a similar thread that shows examples.
DIY reg necklace?
I have the bungie with the slip knots like Splitlip explained, and like the photo's shown by Compudude in the above thread. There are pros & cons to using the knots vrs the zip ties. With zip ties, you don't have to worry about the octo being pulled out. But, if someone for your octo rather than your primary, they'll be jerking on your neck trying to get the octo.
If you make the loop tight around the mouthpiece, there is very little chance that the octo will slip out on it's own. I have yet to have it come out unintentionally (knock on wood).
I don't profess to know which 1 is better, but that's what works for me.