I bought a nice bag that is used by Paramedics. Google it and you will find a huge selection. Mine has extra zippered pouches on the outside of the bag for extra handy stuff.
Google diy scuba tank holder. I found several YouTube videos on making them out of swim noodels mine cost me $3 because I went for the larger diameter noodle Some rope, tubing/garden hose, the noodle and 20 minutes of time and ready to go. Simple and effective. Cushions the tank from the truck bed and keeps it from rolling around. But I will say determining the color swim noodle took time.
That's the way to go. I have 2 of them 25 to 30 bucks off amazon. It is actually a tool bag for a worm drive circular saw. 74 and 100 cu.ft. tanks fit like a glove.