I stand cylinders with caps up in my truck and use a nylon ratchet type cargo strap to secure. I put cylinder(s) in the front passenger side corner (so I can keep my eye on it/them) and hook the strap to the tie down eye, then wrap around tanks, then back to the eye. Ratchet the strap tight and they stay put. I don't engage in any drag racing or off-road excursions while transporting, of course.
I have laid them down before and wedged something under them (wooden wedge, angle iron, etc) to keep them from rolling in a pinch. A "cradle" could be easily made from many different materials to allow bottles to ride without rolling.
NOTE: Acetylene bottles shouldn't be laid down, but if you must, a rule of thumb is to stand bottle back up for the same period of time (or longer) as the bottle was laying down before use so the contents can settle.