Below are the steps used. It is a little long but I did want to get the major steps in.
I have been searching for some very thin saw blades and found a 1.5 mm blade and put it on my saw. I started with this. I have since found a 1 mm blade that I will use for my next one.
1. Starting with a slightly larger than 3/4 in pen blank, I ripped it until it was square at very close to 3/4 inch; I pencil marked the center lines on the end so that I could line up the blank to rip the slots. Double checked and made sure they were precisely centered.
2. With as jig to hold the blank, I cut the edges off and made the blank 8 sided.
3. I set the depth of the saw blade to about 1/4 inch.
4. Next, I set the fence so that the blade would rip the groove precisely in the center.
5. For the first plastic strip, I CAed it in place, It wanted to catch too much and my fingers kept sticking. So I went to plan B for the rest.
6. I mixed 30 minute epoxy for the rest and used rubber bands to hold everything together. (CA is better for Plastic, learned the hard way, more on this below.)
7. After the epoxy cured for 24 hours, I sanded down two side with my belt sander until the ribs were _almost_ flush with the wood, but I did not go all the way to the wood.
8. Next I raised the saw blade to 1/2 inch, set the fence very finely so that the rip fence would allow the blade to remove everything as close to the wood as possible without cutting more wood off. Using the sanded sides against the fence, I pushed the blank through (with two push sticks). The purpose of this is to keep the 8 sided blank as squared as possible. An easier way would be to cut the strips just a minute' tad less than the height of the grooves and that will save some of the extra work that I went through on this step.
9. After that step, Cut blanks into proper lenghts.
10. Place the blanks onto the lathe chuck for precise centering. Drill the holes. Feed SLOWLY. Exit carefully or make the blanks long and cut them off after the required depth is reached.
11. I used small hand cut sanding pads on a barrel trimmer and power sanded the blanks down to the barrels. I did not want to chance the trimmer blades catching.
12. Turn, taking small light cuts. The lower barrel came apart when it caught while turning, but nothing was hurt. I learned from this that CA holds better on smooth plastic than Epoxy does. I took the two pices and formed them back around the tube; placed two rubber bands, doubled, trippled and more - and made them tight on the blank, which was on the tube. Thin CAed everything, rubber bands included, later CAed again.
13. Next day finished the lower part, taking little bitty bites and using regularly sharpened chisel. I sanded the last 1/32 in down. Worked well and finished with CA.
Hope this helps.