Boom Vang
Usually a boom vang configuration has snap shackles at each end. The upper attaches to a boom bail, the lower by the foot of the mast. If your mast base does not have an appropriate fitting already installed, you have several options. Some people mount a padeye to the mast, others bolt one through the deck near the mast base, etc. The 45-angle is technically the most efficient as pointed out, but not an absolute requirement depending on boom height, mainsheet configuration, personal preference, and so on.