For a cheap option, just stop by any optometrist's office and ask for a free glasses glass.
For a SUL option, I'd cut a corner off a tyvek envelope (USPS and Fedex give them away) or cut a tyvek CD-envelope in half and tape it closed. Leave a "tail" on it, thread the tail through a D-ring or other loop on the shoulder strap, double it back, and secure it to the mini tyvek envelope.
Or stitch a single layer tyvek or some light nylon to each side of the shoulder strap with enough slack to hold the sunglasses. Add the thinnest bit of elastic at the opening to retain them when you bend over or lay your pack down.
