Just a thought and you may have already explored this, what about instead of making an entire cap just creating an extension ring that fits into the headlight and receives the original cap on the other end? I would imagine it would be much cheaper that trying to make the entire cap.
Finally got some pictures of it now that I've put the cap on.
2014 Silverado1500 LTZ Z71 Tungsten Metallic:
ARE Z Series Cap
6" Black OEM Oval Steps
EGR Super Guard
EGR In Channel Window Visors
Coming Soon - Weather Tech Digital Fit Mud Flaps.