This 1mm thick silicone cut into strips and wrapped around the glass core for a snug fit.
https://www.amazon.com/gp/product/B08JYYDNZG/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&th=1
With the bottom sealed and slits cut in the glass for air intake, I'm at 5.6V.
Still going strong for over...