Center marking jig. By High Caliber Craftsman. 3:07 video:
https://www.youtube.com/watch?v=IiaZ81OkuZI
Previously:
https://www.homemadetools.net/forum/...271#post172852
https://www.homemadetools.net/forum/...629#post170493
https://www.homemadetools.net/forum/...599#post170413
https://www.homemadetools.net/forum/...326#post169442