Bro, if I'm not mistaken the ss bottles use a silicone seal on the caps and they are known to fail. Maybe use just a 316/304ss cylinder and mount a couple of sight glasses on it. Ebay has some pretty cheap sight glasses.
P,C I was thinking at first about a big gas bottle ones in medium/big fabby, welding shops 300 bar rating, But looking at the whats needed i might look in to some small bottle's on a guys TIG set at work, i think they could be Ally(i don't use these bottles on my plant) there light but rated at 200 bar witch give me the doubt.
As for my general idea of the construction i was going to a standard Esab regulator at the top, witch was fitted to hose that went through a carbon filter but that's an add on, then an hatch or a threaded hole for adding materials and a viewing window.
Anything i missed?
also i have a source on 440 sheet maybe i could assemble from scratch, but that much my cost and that not how i roll
i know you can get carbon fiber ones, you think that could do the job?