If you turned it upside down, looking at angles on the foot, it could screw to the underneath of the boom and become part of the main-sheet arrangement, it looks a bit heath-robinson but would probably work quite well. Is there a similar double block on the track?
I believe it's called an over-the-top block but that may be an Americanism. Used for taking control lines over the forward edge of cabins coamings rails etc.
Well -of no help at all -but there was one in the bottom of a locker in my old twister -and stayed there til I sold it....praps some standard kit circa 1974.....