Find a 0.5mm fountain pen, then pull out the nib and clean up the ink. The nib is just the same size as the aperture of the strap. Then hold the pen tip with a clip and put it in the position where the strap is to be punched. Knock a few times with a hammer at the tip of the pen, and it will be a perfect punch. Remember to put a wooden board under the strap, so that the hole punched out is no different from the original hole and there is no burr.