This little trick I found on YouTube. Involves tying string around where want to split the bottle. Then take it off and trim excess string and soak in some fuel. Then but back on bottle and light on fire. Rotate bottle to evenly heat, then when just about to go out dunk in ice bath and it will split alone your marker.

One thing learned was that need cotton string, nylon will not even come close to working, it melts 😛 I had to get creative to find cotton rope. MacGyver would approve.