Ok, so I just realized I basically repeated what you said towards the end of your post! LOL! Would a channel not be wide enough because the rotation of the arms would make it difficult to get an allen key in at an angle? If so, perhaps fabricating an angled or 'S' shape key of some sort would work? Or better yet, make the butt plate removable and use a 90-degree allen key to reach in over the arms? I'm not sure if you would reach the front two arms but maybe the set screws in the last rear arms would be strong enough to hold it in the open s-foil position. Then the channel on top would be used just to lock all 4 arms in closed S-foil position.
Joe
Joe