It's glass, I'm sysadmin in optical plant
KIS was cheapest start of my modding but surprisingly stable and effective.
The test was with DCT V2 (1.5Om) @4.2V with no-brand-new Efest IMR 18350. It last 13-14h with summary vaped 6-7ml.
This is good result by my opinion. The cut-off is little lower that I want - KIS shuts at 4.75-4.9V and I'v put at In pin diod with drop off 0.8V. And protection form reversed batteries
Till now I'm satisfied with this mod. But never ever ever will do tube mod....if I don't find cheap and good 18650 flashlights!
And maybe smaller LED voltage meter!