If you find that bungie cord or a weighted rudder blade won't do the job, you can also modify a store-bought plastic Clamcleat to release the downhaul line and allow the rudder to kick-up. The cleat has a V-shaped trench down it's middle where the rope gets grabbed by ridges. Normally the farther down you go in the trench, the narrower it gets. The harder you pull the line, the farther down in the grooves it sinks, making the cleat very secure. To release it, you pull up and back on the rope and it comes up and out of the cleat.
To modify it for self release, you drill a horizontal through-hole along the bottom of the trench that's just a shade bigger in diameter than the line being used. That way the ridges will still grab the line in normal use, but if you hit something hard enough, the line burys deeper into the ridges until it hits the new hole and then runs free. It does take some brain work and maybe a bit of experimentation to get the cleat size, line size and new hole size figured out so that it will hold in normal use and release when needed, but once that's done they work pretty well. The drawing below will show you the basics of how they work (the plastic versions of Clamcleats are actually black). I don't think it will work on the aluminum Clamcleats though as the sidewalls don't have enough flex.
