My eyeglass frame mfr. sells clip ons for my frames. You might check with your frame mfr. I carry them in a shoulder strap pocket for quick access.
There are universal plastic clip ons that fold up out of the way that are often sold at convenience stores. They can be cut to the shape of your lenses. They stay attached to your glasses, but can be taken off. They are very practical because they aren't easily lost; however, if you have teenage daughters they'll accuse you of being a nerd and refuse to be seen in public with you, or at least around potential boyfriends. The same reaction you get if you mow the grass in your shorts and white tennis shoes wearing black dress socks.
Do you have a suggestion for a lightweight, but structurally sound, prescription eyeglass case? For safety, I have to carry an extra pair of prescription glasses because my vision is so poor without them, it is my eleventh essential. Just call me