Most driving lights these days actually come with plastic lenses... the ones I have are 2"x6" units with H4 bulbs, for instance, have plastic lenses, reflectors, and housings. I remember when they were metal and glass, and the cheap plastic ones from Pep Boys (Rallye, I believe) would melt with anything but the cheap bulbs they came with that barely lit up anything.
Typically, the new ones are made of the same lens material as the modern headlights. But, you bring up a good point, in that those new lenses aren't made out of standard run-of-the-mill Plexiglas since they seem to stand-up to the heat of the newer-style H4 (and the like) bulbs.
When other things I have going on settle down for me a bit, and after I determine that the H4 headlight conversion kit I have not yet installed doesn't throw enough light, I'll start working on it probably. Sadly, I'll also probably wind up trashing a set of nice driving lights along with a sport lamp housing or two in the process and wind up not actually getting anywhere. But Hey - nothing ventured, nothing gained, eh?