longer and thinner does sound good, i've tested by tubes trying to get them to ring and they just dont, too thick, it's just a clank, stainless is very dense... compared to aluminum. if i had an aluminum tube of the same dimensions it would ring, but since the stainless is heavy it takes more to get it to vibrate, i think i might machine my tubes down on the lathe and see if making them thinner will help the tone.
stans demo cell says 35/40 thousands, outside/inside, 39.37 thousands is a mm. note the inside is thicker which makes sense if you want to design them match tone.
anyone buying new tubes, get them thin!