Jimmy - The patterning plate Daniel is referring to is used to judge where a gun shoots, as opposed to aiding in the counting of shot holes (for calculating pattern density, etc.).
Patterning plates are often used in the gun fitting process, usually in combination with a try-gun. They are typically comprised of a mounted steel plate that is painted or whitewashed between sessions. By having the shooter instinctively fire at a spot on the plate and noting where the gun places the shot in reference to the mark, the gun stock can be adjusted to bring the shot pattern onto the mark.
Daniel - can't help much on the construction. I've shot at a few of them but never built one :-).
Good luck,
Ken
