I get the impression
that Gerard is not talking about how to tension the boom after the sail has been raised, but how to keep the boom from sliding down the mast after he has lowered the sail. Do I have that right?If so, West Marine and every other chandlery sell stoppers that will fill the bill. You need to know the width of the mast slot, and you are good to go. The part is called a "sail slide stopper" or some variation, and they are primarily used to stop sail slides from falling too low on the mast when the sail is lowered. If the slides are allowed to fall too low on the mast, they will come to the part of the mast where the slot widens enough to insert them (and the boom gooseneck), and they will all fall out. However, on smaller boats like yours and mine, the stoppers are robust enough to rest the boom on when the sail is down. A little trial and error, and you will find the perfect spot to locate the slide so that the boom is at the proper height when the sail is furled, and so it doesn't interfere with your cunningham or whatever arrangement you use to tension the boom down.