Here’s the first tube. The glass is a perfect fit! I only had green fishing line on hand but this is proof that a more detailed internal structure will glow and breathe. Shown here powered at 4V. The size is 7mm diameter and 32mm long.

Here is it powered at 12V. I will install eight of these inside Ronaldo’s two HIWATT amplifier heads.

Here are the unpowered placeholder bulbs Ronaldo is using.