We have one at Emmet County that sees quite a bit of use. Steel plate (don't know the exact thickness, but it's pretty thick) about 4 feet square mounted between two 4x4 poles. We use white lithium grease applied with a paint roller. It is useful for estimating pattern, and if you make up a string with a loop you can scribe out a 30" circle around the densest part and count holes if you wish. Just use the roller to smear out the pellet marks for the next shot. Simple and effective, but you don't have a permanent record of your patterns.
