Ken, I don't know how long you have been turning pens, or what you skill level and tool setup is, BUT it is very easy to make a conical shape out of Corian or even better is Delrin, Drill a hoe in a block or piece of Delrin rod, the hole ideally will be at .247 but a quarter inch will do,sorry my drill chart is covered up right now.
If you don't own a set buy an inexpensive set of Dial Calipers, Harbor freight has some quite reasonable, Using the calipers, measure the parts that your blank must meet, Write it down.
Then with the tubes in your blank, install them on your mandrel with a cone at each end small end into the hole, then start turning you blanks, keep checking the size right at the point that the barrel will meet the Nib or Centerband, or Finial, what ever piece you are turning. Now the best part is if you make these out of Delrin CA won't adhere to them and finishing also just became easier, (this is provided you do a CA finish).
You will also find that as your skill and Eye get better your pens will be far more accurate than pens done with bushings. This process is called Turning Between Centers, and many of the guys that turn kitless pens do it this way, Bushings wear out so you wind up with under turned joins, but if you measure your lets say Nib lets say the size is .398 you would want to turn the blank if it's wood to about .410 and sand to .392 which will allow a build up of CA or even some friction polish, ((I would hope not) but you will wind up with a decent thickness of finish, and very little layout for bushings.
I have 3 sets of Cigar bushings, and all 3 are different at some measurable join area (where wood or Acrylic are supposed to meet perfectly)
This hobby is expensive enough with out making it any more so, so get a pair of calipers, find a piece of Delrin, someone on here should be able to help you with that, if not let me know, I can always grab small pieces of round stock.
Turning between centers is by far the best way to create a pen. but always keep some slimline bushings handy, they make great spacers.