I would love to help, but what exactly is the problem?
Thank you. I don't know how to length the tube and still have enough depth to hold the top or bottom section. When that has happened to me, I tried to find a kit that take the same diameter tube and nearly the same diameter blank/materials. I then shortened the tube to match the other hit and turned the blank down or built it up slightly with CA.
Sorry for not having a solution.
Stephen,
Try this, drill out a black piece of wood or acrylic, then use the bushings and turn and finish.
Now you can slice off a ring the thickness you need, remove the nib and put the ring in.
You should have enough hold if the ring is only a couple of mm´s !!
Irv