It may be possible to adapt a lever type boom vang and get by with the single backstay and chain plate. Assuming a masthead rig I would avoid any block and tackle rig that wasn''t fail-safe. The simplest rig would be a stout lever with its end shackled to the chain plate and the backstay fitting attached to the lever a few inches toward the other end. The "other end" would carry a three or four part tackle to a cleat on the stern quarter. You would have to work out the mechanical advantage needed. And it is IMPORTANT to make sure the total geometry didn''t allow unfair strains on the chain plate or any of the backstay fittings through out the entire possible range of lever movement. This is offered as a concept only. The engineering needed is up to you.