Any sized material can be used, the important thing is to space the center hole exactly at the mid point of the two outside dowel holes. Use whatever size dowels you have, the ones shown are 1/4", drill the hole for the screw slightly smaller than the diameter of the screw.

Insert a wood screw to just protrude through the bar, you may have to touch up the point of the screw with a file.

The dimensions shown are a good size when working with 3/4" and 1 1/2" thick material. If you are working with larger material just make a gauge with the dowels farther apart than the thickness of the material, the dimensions are not critical. The screw can be replaced with a pencil if the bar is made from thicker material.