This wont help averybody,
A buddy that builds a few bikes does this. Builds his own lenses. then puts LED,s behind them.. I think lenses are just glued in, will have to ask. At first they were sort of bigger, maybe signal light size, but eventually he got it to the point where they could fit in the leading edge of the fender ! (great for bobber, not cafe maybe).
Anyway, full functioning signals and stop lights in a very clean package, some folks think there are none on the bike till they get lit up to show you.
I know they used to use prismic red lense material from the sign industry..looks like a piece of a flourescent light cover, smooth one side, knurled look on ther other.