a cheap way to do it would be to find some black tape and put that on. a guy with a body shop showed me a e30 that he had done this on and it looked fine. might sound like a bodge but done correctly you cannot tell it is tape
We get asked about retro fitting the black optics kit from time to time - whilst technically possible it would be an absolute wker of a job and take a hell of a lot of labour.
Same sort of paint or vinyl coating I think is the only practical option.