Calvin - I definitely like your idea.
As proof the concept works , years gone by Kodak used to sell wratten filters - a gelatine substance one could cut to size and place behind the exit lens element and in front of the gate on 16 and 35mm cine camera's.
I often used ND filters that way.
It would be great if they were still around cos I would use them.
The idea of no image degradation due to ND glass in front of the lens has got to be a good thing.
Re your approach, I'm not sure the optical quality of your sun glasses cut out is up to the demands of out sensors and lenses - unless you know it is.
BTW, nicely cut out sunnies😎