We use a lid from a dog food container that we beveled out a small "dip" for the eggs to sit on and then a very small hole to shine the light source through. We currently use a flashlight and it works great! We use it for our leopard Gecko eggs mostly, but since we have 46 Veiled Chameleon eggs in the incubator, we have used it on them as well and it works wonders. We have only done it to the Chameleon eggs a few times and used a different egg each time. We are hoping our eggs hatch out in October, but it could be as late as January!! They were laid April 1st.......what a kick!
I found a tiny light made for reading in the dark which has a clip on it to clip it to a book. It works very well and the bulb is an LCD so it doesn't give off any heat. I use it without even moving the eggs from the container. It doesn't work well for determining how far along the embryo is but you can tell when the egg is out of diapause.