For $20 you've already spent, try this:

Tape the spine.

400 grit sandpaper on a flat stone or even a counter top - put the edge down like you are cutting bread and then instead of sawing parallel to the edge go at about 45°.

Grind away the frown.

Hone it up starting on the 400 grit.

Worst case scenario, you've just wasted $20.

Mark