I've had great success with a Dremel and a flex shaft. I just put a small diameter sanding band, or grinding stone bit in the flex shaft, and use that to grind down the end of the hook. You just have to be careful inserting it into the motor mount, cause if you miss, it'll chew through the mmt tube pretty quickly. If you don't have access to a Dremel with the shaft, a good small file will work, as you said, with some patience.