This is my version of a center finder. I like the simplicity of the 2-pin units over the parallelogram devices. I also like the idea that nono30 had by storing a pencil in the end of the device (see https://www.thingiverse.com/thing:2594439). Well, that gave me the idea of adding a pencil sharpener to the end of the pencil storage hole.

You can use any pencil that's about .300" diameter. I positioned the sharpening blade at 20 degrees (golf pencil taper) so that more of the pencil body is in the centering device.

This unit is long enough to center a 6" board (5.5" nominal). Any larger than that, and you should take out your tape measure!