Zenni ships their glasses in a snap-close case with a fuzzy insert on the bottom, but after you unpack the cleaning cloth and suchlike, the glasses rattle against the hard plastic top.
Make trial fit prototype from thin cardboard and trace it onto a sheet of craft foam:
The pen, much favored by quiltists, has a white ceramic lead that washes out of dark fabrics. You can find a corresponding dark-lead pen, but I can use an ordinary pencil.
Use different colors for different glasses:
Then walk ninja-style again.
Protip: slip an address label atop the foam so a nice person can reunite you with your glasses, should they slip out of your pocket in the unlikely event you sit down anywhere other than in your house.