FWIW our experience with a home made boom brake has all been positive. It is not a preventer, but it slows the boom and can even bring it to an easy stop by hand tensioning the slack line. It serves a vang function beyond the end of the traveler without loading up the gooseneck fitting. Its reason for being was to prevent the possibility of the boom lifting and fouling the backstay since we don''t have a boom vang.
To jibe in significent winds we sheet to centerline then ease out with the brake at a faster rate than is convenient by tailing the mainsheet. This prevents loading up the helm and the running mainsheet(4:1)doesn''t burn the hands.
The symmetrically rigged control line runs forward from the cleat at the cockpit, to a block on the rail about 2-feet abaft the shroud, over the brake''s drum (4-inch dia.) with one or two turns on the drum then out to the other rail to a cleat aft. One turn on the drum doesn''t inhibit tacking in light air. The boom brake is attached to the boom 5-feet abaft the mast. This has been adequate for a 283-sq.ft. main on a 14-foot boom.
The brake''s construction in aluminum was simple and straight forward. It''s a basic model without the tensioning ''bells and whistles'' found on the purchased item. I''d be happy to detail its fabrication to any one interested.