The easiest course might be to install a rigid vang, simply because they usually come with custom mounting hardware.
I'd start research with Garhauer, and you call call them on the phone to discuss.
A vang kit, meaning a tackle, may be somewhat cheaper.. Specialized fittings for the mast are available. The fitting on the boom is usually just a bail, as you have for the main sheet. Some sort of stopper needed to keep the gooseneck in place.
Vangs are a "modern" invention. When marconi-rigged boats replaced gaff rigs, they had long booms. In a jibe, the boom would rise up and take out the backstay. Coincidentally, yachtsmen realized they also suppressed the awful rolling of narrow, deep hulls when off the wind.
Voila! The boom vang, one of the great nautical words with which to confound the unwary landlubber.