As I noted in my comments, the drawing is a bit misleading. The line should attach to the boom further aft/outboard. Mine is a bit aft of where the rigid vang attaches. There is plenty of leverage to hold the boom in place.

I use a non-stretch line, around 9/16. The main point is to keep the boom held out securley to prevent a jibe in the first place. If one were to happen, it would be somewhat 'controlled' since the boat itself would have to gradually turn. As the wind pipes up I tend to reef the main early, so I'm dealing with a sail sized for the wind.

I have been on lots of boats where you have to crawl way forward to attach a preventer line, or hang out to attach to the end of the boom. Those discourage the use of a preventer, and adds to the rats nest of lines which makes things more difficult especially at night.