Resistors are a wonderful invention - if they work for you, use 'em.
Richard: I went that route, the milky-lens version, a lot of years ago. I love the look. I think it's Pre-A, but I have a white car and the milky lenses look awesome.
That said, the lenses I got were hideously expensive at the time, From Stoddard's, IIRC, and are made of glass. Still, here is what I'm running for bulbs:
These bulbs are very short profile incandescent bulbs, painted with amber paint. They are very hard to find so a couple of boxes of spares were bought and stored, when I found them, in the parts bay. The bulb # is a GE 97A
Because the bulbs are short(er) they don't get close to the lens and probably won't be close enough to melt it. Also, my directionals are only on when the directional is flashing - they are not running lights and, most of the time, they're off so they've never generated much heat. Just to make sure, I spaced the lens off of the base an extra 3/16" by using a home-made ring of closed cell foam between the base and the lens. It's waterproof and pushes the lens out just a bit while still looking stock.
Hope this helps. Here's the look. You can just see the amber bulb behind the lens:
That's an older photo - I now have a pair of headlight stone screens like you!