UltraLight gear screams simplicity and utility. Extra features just add unnecessary weight, so we avoid them like the plague. This Make Your Own Gear project is about as uncomplicated and inexpensive as they come, and it works great too!
Start with a flat piece of cord - I like to use this type I get from zpacks.com, but you can use anything you have. Cut it to about 26" long, then singe the ends.
For the end attachments, I tie a modified taut-line hitch. The difference is that you only wrap around the cord once in the first step to keep the knot as small as possible.
Just slip the loops over the ends of your sunglasses, snug up and you're done. A .10 ounce super simple project that will last you for years and years.