I spray most of my pens with acrylic urethane. I cut the mortise and notch before I finish the pen. If you have an old 1" or wider bandsaw blade, or can get access to one, then you can modify the blade to become a recess tool. Just cut an 8' section, grind off most of the teeth, grind the last tooth to be the width of your clip ring and a small bevel on it to be a cutting edge. You need the cutting tooth right on the very end, so the tool rides the side of the tube cutting the recess. Learned the bandsaw blade trick in an inlay class I took years ago. The blades are pretty ridgid and made of tool steel. They are easily modified into cutters and scrapers using a simple bench grinder.