I use a small LED flashlight. It's one of those purse size, has several little LED bulbs, and was really cheap to buy. The end of the egg completely covers the lens of the flashlight.
That's what you need is no light leakage around the egg. In a dark room, that is plenty of light for duck eggs and even for the thick shelled goose eggs.
It came in a package with 3 little metal flashlights and was purchased at one of the Big box stores. I can't remember if it was Home Depot or Lowes.