I'm going to tie my bight off to this section, and rather than just securing it to the horizontal ropes. I'm making sure to go also around the verticle part that is holding the wrists up. This will prevent the ropes from loosening unintentionally as the knot might slide a bit with tension if only fixed on one axis. The knot is a simple overhand knot. You could also use a lark's head - I often do here - but by knotting this rather than running your working ends through the bight, it is easier to undo or shift around later.