The drill press creates round holes, and like most mortises, we needed a rectangular hole. Using a sharp chisel it takes only a few minutes to remove the remaining material to make way for the portafilter shaft and square out the corners.
There are many more ways to make the mortise, including using only a chisel from start to finish or a mortise attachment for the drill press. This was just the quickest and easiest way for us.
Keep removing material with the chisel until the mortise fits snug on the portafilter shaft.