As seen at the prop summit, here's how I built one of these!
A quick reminder of the reference from "The Child" and "Suddenly Human." Not a lot to go on - there are a few more frames (not shown) in the scene from "The Child" that hint at a switch located in the small end recess of the highlighter body.
So, first step was to gut the marker - pull out the nib, cut off the black molded tip where it begins to taper to fit into the cap (leaving a black band), and remove the ink wadding. There seems to be a raised detail on the side of the thing below the clear tip, so I added some styrene in the approx size. Also drilled and filed a square hole in the end recess for the miniature slide switch (a push-on push-off latching pushbutton would work here too).
Next step, making the tip: When making the master, I took advantage of the now unnecessary marker cap to save time since it's already the correct shape in cross-section. I removed the inner piece of the cap that seals around the marker tip, filled it with resin, and then used a belt sander to create the shape that I see in the reference. I also taped off the wide end of a marker body, set it standing vertically on the bench, and back-filled about 1/8" of resin into the wide end (through the switch hole) - this plug was glued to the bottom of the tip master so that it would friction fit into the marker body. The cap and plug could have been filled at the same time by taping them together but I just didn't think of it.
The tip was then molded for casting in clear resin. A small hole was drilled into the cast clear tip to hold a warm white LED at approximately the depth seen in the screencaps. I did some experiments with tinting the resin, but painting it with clear blue (Tamiya X-23) worked the best to approximate the look.
A little grey paint, white & black vinyl stripes, and it's done!