Black PETG definitely looks better than cyan for this job:
Holding the plate cap to the tube with a thin ring of opaque epoxy cuts down on the glare under its edge:
Fire in the bottle!
It’s still running basically the same Arduino code as before, but I have some ideas about that…
The Smell of Molten Projects in the Morning 
The reflection of the surprisingly regular mottling of the lower mica is unexpectedly nice.
The mica sheets in those 21HB5A tubes look like they have a powdery white coating, quite unlike the usual shiny dark-silver mica sheets in my other tubes, that lights up wonderfully well under the LEDs.
Peering at the bottom of another 21HB5A, I think the mottling comes from refraction around the pins. Each pin has a glass cylinder melted around it; they may have put a sealing bead on each pin, combined those with a glass tube base, then melted that whole assembly onto the tube envelope. Plenty of forgotten tech involved in that process, fer shure!