I am now having complete end caps and spacers machined.

What I wanted to do was try and keep it simple.
The problem with the cell, or the hard part, is keeping the two tubes correctly spaced.
Inserting three pieces of plastic down inside the tube was difficult to do and was inefficient.
It also would fall out and was not serviceable in the long term
There needed be a cheap and easier solution to the problem - I hope this is a start in the right direction.

Each tube can be purchased individually and combined to make a cell with as many tubes as desired.

316L grade stainless steel in two sizes.
25.4 mm in diameter, 210 mm long.
19.05 mm in diameter, 250 mm long (so it can be fitted to the inside of the 25.4 mm section).
The resonance cavity between the two tubes is 1.57 mm.