The original plate cap, even without fins, seemed entirely too large for the 21HB5A tube. There’s not much wasted space inside and, after trimming the outside a bit, this is about as small as seems possible:
PETG doesn’t bridge well and, after cleaning out the wire hole, the remaining shell didn’t hold the brass tube very securely. Epoxying tubes into two caps at once, with a longer brass tube holding them in alignment, worked well:
The tube eliminates vertical tilt and you (well, I) can eyeballometrically align the caps and tubes in azimuth. The thin ring of JB Kwik epoxy around the brass tube isn’t visible, so it’s all good:
This project may eventually force me to try epoxy coating, high-build primer, and good paint…
The Smell of Molten Projects in the Morning 
For a moment, I thought you were aiming for a steam punk push-pull amp…
Jim R
Now that is a great idea!
Some casual searching produces a great example: coppersteam.com/gallery/steamamp1/