The bolt on the inboard lip of the hull is an "active" attachment. Whereas the beam sliding around the casting is a "passive" attachment. I firm that (outboard attachment) up by drilling two hole though the beam and casting on each side. Then rivet the beam to the casting. PLEASE note if the casting is loose this won't do squat. But if it is firm it will work well.
I never realized how much that beam migrated on the casting until i was sailing while sitting on the wing with my foot resting on the rear beam at that point one day.
The rivets helped alot. Also the previously mentioned tricks to "shim" the underside of the beam (wether 5200, microballoons or just an old square of neopreme) help alot also.
