A simple device for finding and marking the center of a dowel, my lo-fi version of a similar metal device I once used in a wood shop.
Hold the dowel with one face flat against the raised diagonal rib, and the sides against the vertical walls of the center finder. Tap the exposed face of the dowel with a mallet to score a line across the opposite end, then rotate 90 degrees and tap again to score a second line, forming a cross. The center of the cross will be at the center of the dowel.
This is a 3D-printable version of an existing expensive metal tool, and will need to endure a fair amount of abuse, so print at 100% for best durability!
The STL is sideways, and is designed to print with the base on the print bed. Rotate the model so that the face with the two holes is sitting flat.