I made a boom brake out of some rope and one of those belaying cleats
you can buy in a shop that sell gear for abseiling and mountaineering. It is alloy and shaped like a figure 8 and a good one here in Oz sells for about 30 bucks.
I ditched the boom down haul, ran a line
from one chain plate through the belaying device ( sorry I dunno what they are called) which was attached to the boom at the old mounting point for the down haul about 4 feet back from the gooseneck. From there the line
runs to a block on the opposite chain plate and back to a cleat
on the outboard side of the cockpit coaming.
Simply tension it as required with a winch
it off. It won't prevent a gybe but slows it down significantly. The heavier the line
the greater the friction generated. I have found 12mm to be just right.
Because of the attach point fairly close in to the gooseneck, it is probably unsuitable for a boat much bigger than my 30 footer, but so far it has worked a treat.