[kwerk]

Given that you wanna go from a standard size neck pu hole to a bucker, you could be scraping or filing for a long time. Another option would be to mark out your bucker area and drill out the remainder. Leave enough to clean up with a file. In theory you should be able to get your corners perfect with this method if you select the right drill bit, then simple straight line cleanups after that. You could then even cut from corner to corner with a craft knife if necessary.