http://www.aurorasilk.com/shop/hemp_yarn.shtml
Here are some clothing-quality hemp yarns, but I have no experience personally with them.
Knitty has a guitar strap pattern, super simple looking too....
http://www.knitty.com/ISSUEfall03/PATTbanshee.html
I'm sure you could leave off the embellishments if that's not your style.
