I've got a set of the expensive harness lines, yeah I balked at the price (these ones?)
Yeah, not cheap. I almost went with a much cheaper rope set (like what you've got I'm guessing), but when I saw how they worked, I figured they'll be better in the long run.
the key thing that makes them better is they run the rope line and tube back behind the stiffened material so the rope can't twist up into a coiled up unhookable mess. Maybe you can recreate this to a degree by making a stainless steel bracket with a 90 degree bend in the end, with a hole drilled to fit the tube, that attaches to the cleat at the other end, with the distance restricted to the length the rope goes out to so the tube can't slip out. That way the tube can slide up and down as required, and keep the harness lines rigid. Maybe a dud idea.. I'm not an engineer.

if Patrik (or any other manufacturer) made a cheaper version, with just the rope adjustment, but the same piece of stiffened material all the way down, I'd have bought that instead.