When transporting our fifth wheel trailers 30-pound LP-Gas cylinder to get refilled I like to use a classic 12″ x 12″ inside diameter milk crate to help keep it upright. The cylinder fits snugly inside. Then I utilize a ratchet tie down strap to hold it against the truck’s bed rail.
I find the milk crate handy around the rig in other ways. For example as a step stool, storing loose items, trash bin, small table, etc.