I had the same problem on (what looked like) the original 1979 boom. The track was badly worn from years of sail changing and adjustments.
The Velcro strap worked great. Mine is about 24" long and wraps through the clew and around the boom almost three times until...
I ended up replacing the boom with a boom tube from RigRite because the old boom had arrived from the previous owner with, er, a "slight bend" at the vang plate. (There's another, much longer story about me driving across the Canadian border at 11PM with an aluminum tube on the top of the car and no paperwork which I won't go in to....).
I moved all the hardware and rigging over from the old tube except for changing the Boom-Vang attachment point from the old surface-mounted style to the "thru-boom" style that Rigrite offers (Internal Boom Bail: K-11428 on their website
here ) which requires a slot to be cut in your boom.
I also cut the new exit that Russ mentions and would HIGHLY recommend it if you have a loose-footed main that you roll off the boom each time, makes for very happy crew.
