This is a simple device for finding the center of a dowel or other round piece of material, and is my lo-fi version of a similar metal device I once used in a wood shop.
3D printed at 100% infill for durability, because the way this works requires it.
Place the face of the dowel against the raised diagonal fin, and tap the other end lightly with a hammer. This will score a line on the face. Rotate the dowel 90 degrees and repeat. This will result in a perfect cross in the center of the round material.
Download the file and print one for yourself on Thingiverse.