I'm not sure the shape of the hole would matter that much as the weight should hang by the base of the handle. If there is an issue with how well they stay on the rack then I would embed a 1/4" rare earth magnet in the body of the holder behind each blade. Lee Valley sells them cheap by the 10 pack and someone else posted a source for 100 that was dirt cheap.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.