I recently backed this project: http://www.kickstarter.com/projects/2026249294/drawstring-backpack-reinvented

And its great, I absolutely love it! However I had one major gripe about it, the cord would slide off of my shoulders. Because of that, I created a a clip that would hold the cord together preventing and slipping. Hopefully this will solve and slipping issues that you may have!

Printed on a Replicator 2:
2 Shells and 6% Fill
0.4mm Layer Height

You can print at a higher resolution but the settings mentioned above are perfectly fine.