New to the forum here. Every time I go in to update my prescription (I wear glasses only), I get a new pair of glasses & tuck away the old ones somewhere useful, always keeping the most recent pair in the car. I also keep a few anti-fog lens wipes in all of my toiletry kits, though plain soap & water will work fine. I used to keep one of those cheap repair kits in my BOB too, but realized none of the screws would fit any of my glasses. Duct tape worked fine the one time a set of glasses fell apart, at least until I got to a fresh set.

I've also used duct tape as a glasses holder: During a weeklong offtrail hike, I realized in the first few minutes that I'd forgotten to bring a sunglass holder to keep the brush from ripping my glasses off. Can't live without duct tape!

If space in an issue, like an EDC kit, I had a set of prescription lenses fit into "optical quality" folding reading glasses. If the folding frames are too cheap, the optometrist won't like putting prescriptions in them. I think Myspex brand was the one I settled on at the time, but I think there are some less expensive ones out there. When tucked away in their case, this spare set is about the size of a small & fat cellphone.