This one is made from a slim kit, but the process and concept are the same:
Using a mandrel (or your favorite mandrelless method):
*Glue the tube to your blank and square it off.
*Mount it on your mandrel and turn round.
*Pare off some material at the CB end. There's no rule here, make it as thick or as fine as you like. (You do this on the lathe so the cut is square to the pen.)
*For the one above, I took some .032 aluminum and drilled a 7mm hole. I also took a scrap of black PR and did the same. The PR is a bit thicker than the length of tube visible.
*Take the blank off the mandrel and using epoxy or CA glue, slide the aluminum and the PR on the tube. Clamp.
*Square off the end again.
*-- NOTE -- You can leave some of the CB hanging over the edge if you need the space for length sensitive kits.
*Mount back on the mandrel and turn like normal.
It's really easy and opens up whole new designs.