Rather grandiose for a Laser 13 admittedly but this arrangement allows for great flexibility in raising/lowering the boom when cruising, which avoids the problem of having the whole caboodle dumped in the cockpit when the sail is lowered.
I fitted a small pulley at the masthead, attached to a bracket riveted onto the mast. I placed this to one side - starboard, as it happens - to avoid the main halyard. The image shows the mast lying down for transportation - obviously a bungee cord doesn't normally go around it:
Through the pulley I ran a thin line which is about double the mast height in length. When not in use, both ends of this are tied off at the foot of the mast. In use, one end is simply attached to the outer end of the boom. In the next picture you can see how it works: tightening the mainsheet stops the boom waggling around.
Not furled! The whole thing would look far neater of course were it not for the flotation pad fixed in the top of the Laser 13's mainsail. In fact the efficacy of this is so doubtful in my view that I have been tempted to remove the pad entirely, though that would involve cutting open the triangular 'pocket' in the sail. Here's another, on the River Colne: