I'm kind of a blender noob and need some help. The picture below sort of sums up the problem. I have a pair of black glasses which I did not model. I also have some "lens models" that are 8cm in diameter and modeled to the exact shape of a specific eyeglasses prescription.
The image below is an exaggerated solution, some of the prescription lenses are a bit thicker than the frames, but not this much.
The obvious solution was to select a loop of vertices on the glasses and extrude them out into a mesh I can boolen with the prescription lens to cut it to size. This didn't work because selecting loops on these glasses didn't seem to work. So how to do it?
The prescription lenses are actually like real glasses lenses, I have to use them to get correct refraction when this is ray/path traced, so I need a way to cut them to fit the frames.
The model for these glasses is available here. It already has some lenses in the frames but not all models I want to use do, some are empty.