Yesterday I rigged a preventer to use when running downwind. It consist of about 30 meters of 8mm polyester braid line(stretchy) that runs all the way to the stem (outside everything), through a snatchblock and back to a jammer on the cabin top.
I can control the preventer from the cockpit and put it on a winch to set and (most importantly) ease off gradually when necessary. Sunny Spells has the main sheet attached to the end of the boom, and the preventer is attached at the same point, using a snap-shackle. This prevents bending moments being applied to the boom by the preventer and main sheet working in opposite direction, resulting in a broken boom… Fortunately this arrangement is also good at preserving the boom intact should it get dragged in the water!
At 25 meters, the preventer is long, almost double the required length. I did this on purpose so that I can just throw off the jammer when changing tack without worrying that the line will pull through the clutch. I just leave it shackled to the boom and re-run it after the gybe. Also, the snap-shackle stops the line from pulling through the snatch-block on the bow. When I need to re-set (or stow) the preventer, I just pull it through in the cockpit, flaking it on the cockipt floor, until the snap-shackle stops in the snatch-block on the bow. Now I go forward, swap the snatchblock to the opposite toe-rail, take the snap-shackle (outside the lifelines) and walk back to the cockpit, pulling it through as I go. I just snap-shackle it to the boom end, close the jammer and pull it tight – too easy.