Rigging question, in the small.
 

Strangely, this seems the right place to get expert opinion on this, though it has nothing to do with boats, except that it unburdens a sailor from a shoreside chore. My mother has a pair of spectacles that uses monofilament to hold in the lenses. Each lens has a circumferential groove in which the monofilament lays. One lens has a tendency to pop out, and the various optometrists she has visited have failed to correct this. Looking at the spectacles, I see the problem is the shape of her head, which gradually has put the two temples at slightly different angles, and thereby caused an unfair lead where the monofilament starts on one side. It thus enters the groove at an angle. I suspect this is where, when the glasses are stressed, it has opportunity to jump the track, so to speak. I've bent the frame a bit back into alignment, but suspect that won't last long, since spectacle frames over the years tend to fit themselves to the wearer's head, for better or worse.

Any suggestions to keep the lens in place?

It's hopeless
 

Hi,

I also had a similar pair of spectacles many years back, and my lenses came adrift almost daily. You might try super glue, but if that fix should break, you'd be done. Trying to fix a poor design can be the ultimate frustration; I remember trying to make a spade rudder stop wobbling in a heavy following sea. The obvious solution was to get a full-keeled boat (which I did). Luckily getting a new pair of glasses is far less expensive than getting a new boat.
I've noticed horn rims are making a comeback.
Good luck!
Ben