A Dremel w/ a sanding drum bit. Pass it slowly and lightly down each side, lightly because you don't want the bit to dig in to the foam, run the bit down the foam going w/ the rotation of the Dremel or again it will dig in to the foam, After each pass check to see where the edge needs a bit more "sharpening"....This was just a test piece but I got a lot out of it on what to do better next time. Thank you for you interest. Let me know if you need anything else....