William Duffield
I wouldn't use a farrier's rasp, because it is way too coarse, and the bottom inch of the board with the knot will be exposed in the finished case. In fact, I would be reluctant to use one of my hand stitched cabinetmaker's rasps because I would still be afraid one or more errant tooth scratches might be left exposed, if I wasn't careful enough.

I think I like Mark's idea better, or at least go a couple steps up to a #7 (or #607).