I also had a problem with my Nacra's rudder down haul line "creeping...losing the rudder rake. My simple solution was to use a bit of whipping line, placed right where it would jam in the jam cleat, when the rudders are fully down.

The whipping line (like one would use to whip the ends of sheets, down-hauls, and halyards) is wound around the rudder down haul a few wraps and then I use a sail maker’s sewing awl to sew a locking stitch through the down-haul line.

The approach simply increased the downhaul's diameter at the one small spot that is critical to help jam the line in the jam cleat.

Works well, it's cheap and fast ...

Good luck

Tom H.