A simple and cheap boom brake...

Some time back I mentioned various ways to deal with boom brakes and how a simple figure eight descender/belay device works just fine... Over at Sail-World they have a great article showing you how to do it.

I'll add that instead of rigging it as they describe with one end of the sheet dead ended to a chainplate it is better to have both tails of the sheet come back to the cockpit so you can put equal pressure on the system and gain a lot in the way of control at the cost of an extra block and a couple of shackles. We've done it both ways and the twin tail system works a whole lot better!