I went to our local cooking supply store for equipment to make a modification for a photographic flash. It was not as esoteric as you were probably thinking. Just a couple hundred black straws.

You see a flash creates a lot of spread out light. Even when you manually zoom the flash as narrow as it will go (to 105mm).

However, if you put a lot of long straws in between the flash an your subject, those straws act as a limiting element. You switch from a wide rectangle to a small circle. Neat, huh?