I used lilac sharpie on some thin plastic film. Then I cut it out and glued it to frames behind the original lenses. I had to quickly rub over the sharpie whilst it was still wet to get out as many of the lines as I could, it's not great but it was the best thing I could come up with in the time I had (I'm still working on a better solution).
I'm working on a AC Riddler myself,Just looking for some lens material. I was thinking maybe..you know that seethru plastic in visors on those hats? come in different colours,maybe that might be a goer..I'll scope around anyway ^^