I wholeheatedly agree that on a cruising sailboat, particularly with inexperienced passengers, there is rarely any good reason to sail dead downwind or to not have a good preventer rigged. Here is a simple rig involving minimum rope and hardware: Dead end a line a the aft end of the boom long enough to reach the shrouds with a quick disconnect fitting at the forward end and a bail on the boom to store it. Run another line with the same fitting from the point at the shrouds through a swivel block at the bows and back to the cockpit. To set it up, sheet the main in to where a man can reach through the shrouds and connect the two lines. Before you jibe, come up on the wind, sheet in, disconnect, restow the line on the boom, jibe, take the same end of the preventer to the new lee shrouds, and reconnect. The hauling end does not have to change sides.