I use a small LED flash light and my hand. It does not work on the dark ones but does allow me to see most of mine well enough to know if I am getting something. IF I can wait until day 7-9. If I try earlier, I'm just frustrated.
I just candled and tried out my new candler. It worked okay, but I think I'll stick with the flashlight. I pulled 8 infertile eggs out of 42, so doing pretty good. All the other eggs were doing great!!
I'm guessing you're using the container to separate chicks of different breeds. How tall do the sides have to be to contain them? (I usually hatch big, clumsy orp chicks who like to play soccer with the unhatched eggs. Any problem with air flow or temp changes when the container baskets are added?
To candle eggs we use a bright 9 LED pocket flashlight. Many have a soft rubber rim, so they're like a real candler. Because they're inexpensive I have about 5. This way several people can candle at the same time.
They don't work on the Marans or Welsummer eggs (only can see air cells), but reg brown eggs are fine.