philil's suggestion to make sure that you don't cut or sand off too much and damage the riser incert is good advise.
one way to be sure would be to measure the dept of the hole and then apply that measurment to the outside of the riser.
take a round file or anything that is round and less than 1/4 inch in diameter and incert it all of the way to the bottom of the hole and pintch it at the surface, pull it out and then lay it along the outside of the riser to see just how deep the hole is. Make sure you don't get anywhere near it when you do your cutting and sanding and you'll be fine.