I have this one. I use it to carry my Leshiy 2 350. I remove the quick detach behemoth and fit nicely. Instead of the behemoth, I think a 500 cc cylinder would fit along with the gun. Maybe both the behemoth and cylinder? Just make sure it’s the 15L pack as shown in the picture. I also have the smaller 9L pack but it’s too small for the L2. It’s perfect for the original Leshiy though.
VERTX packs are nice I have a vertx Gamut 2.0 And a vertx overland Lots of pockets storage , Molle webbing and they sell aftermarket upgrades and accessories to add or attache stuff ,the best thing is they make lots of color options and don't look tactical from the outside. https://vertx.com/gamut-overland-ba...2FGf715LanNB3pNdP65ZocoiW8CKr4EoaAoCvEALw_wcB
They cost about $120-160 avg. Also look on ebay ,they have some available
This Specialist Covert case is designed and tested specifically for professionals like you in mind. Opening the muted cover reveals endless options to build this case to your exact needs.