good point, I use metal jerry cans, or plastic race jugs. Not those cheesy plastic spill masters

Examples:
https://www.amazon.com/FAIR-TRAILS-...ocphy=9061181&hvtargid=pla-437430502521&psc=1
https://www.amazon.com/Wavian-Authe...UTF8&qid=1542660918&sr=1-4&keywords=jerry+can
I don't like carrying anything explosive without it being strapped down. Propane tanks, and heavy 20lb Co2 tanks are other dangerous item I tend to transport often. For now I'm smashing them between the seats and packing sheets or blankets around them. Not ideal though. It could cost me my life if I got in an accident with a 20lb C02 tank not secured, those things are heavy!
Both my ford focus and volt both had tie down hooks in the trunk, so I would use tie down to secure the items against the seat. (granted they are both hatch backs, but I don't see any hooks on my loaner S either).
Any cooler seems like it would be problematic as it will slide or roll around in the back as well, unless your packing a ton of extra things around it.
You never know when your going to have to lock on your brakes or swerve hard to avoid an accident, or worse yet to be in one with explosive, heavy objects in the trunk not properly secured.