If you have enough thickness to use a bearing guided roundover bit you could lay the holder on its side and run it around a table mounted router very carefully. Because the bearing would limit the depth of cut, it would allow you to stop and restart, giving you a chance to keep moving your hands to a safe position.
If that proves to not be an option, the next best way I would try is to take a sanding drum on drill press, or clamp the knife holder and use a drill or dremel mounted one and freehand the curves on with light passes. A little light sanding after will finish it up.
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.