Simple holder. On the solar light, the diameter is around 14 ½ to 14 ¾ mm. Across the ribs, is 16 ¼ to 16 ½ mm. The walls on the holder are very thin to allow it to deform to the ribs for a light press fit. The lights I used was a 30 pack from Amazon. They look a lot like the ones from Dollar Tree.