If you're trying to avoid plastic containers, working glasses with lids like these from Crate and Barrel or Luminarc are great for food prep storage or small portion leftovers. I have these jars in the 14 and 21oz. They can also double as wider drinking glasses. The lids will probably fail over time, but the replacements are cheap, and the sizing specs seem to have stayed the same for decades. For even smaller storage to hold things like sauces/dressings I use 4oz Ball canning jars.