The boom downhaul and the cunningham are two different "devices". The stop in the megigharbor picture keeps the boom from falling/sliding down the mast when the sail has not been raised. When I sailed my Cape Dory I always loosened the boom downhaul, raised the sail to the top of the mast, and then adjusted the downhaul to tension the leading edge of the mainsail. The cunningham is a different line which is usually dead ended on the mast, passes up through a cringle in the mainsail and then leads down to a cleat on the mast, a cam cleat or other quick release cleat. This lets you adjust the "belly" of the main if necessary depending on wind strength and the tack which you are on. I hope that megigharbor or someone with a movable gooseneck can expand on this reply. Truth is, with my fixed position gooseneck on my Mod 3, and putting significant tension on the mainsail downhaul to tension the leading edge of the mainsail, I rarely have used the cunningham although it is in place and left with enough slack to have no tension on it. If racing I would be a little more concerned about adjusting the main. Same thing with the outhaul on the main. I tension it before raising the main and don't worry about adjusting it while sailing, which would affect the curvature of the loose footed main. Again, I am not racing, simply enjoying sailing a delightful little dinghy.
Good sailing!