I kind of cheated, cause I'm cheap. You can get those foam floats for boat keys. They are a couple of inches long and oval like an oversized medicinal tablet. They have a hole in one end.
I take the leach my glasses are on and "slipknot" the float to the middle. They will float a heavy set of glasses but you won't notice the weight of the float.
The slipknot I use is to fold the leach in half and stick the fold through the eye of the float. Now take the two loose ends and slide them through the fold of the leash.
Here's an idea The link above has the floats I used. They are in the upper left corner of the add. Oval foam floats. Comfortable, cheap and no emotional attachment. Kind of like the ladies I dated.....