Morning Jerry;
I think the foam between the weight and the plywood is the issue. Allows too much movement of the weight. Hard fasten with glue, double sided tape, and then the screws. Other things that I have done too is to install diver lead bb's inside the rudder. Simply drill a hole in the bottom of the rudder, fill with the bb in the amount of weight, then Monokote the hole shut. Other things that I have done is to glue the bb's in place with epoxy. First using the bb's rolled up in a small plastic bag or something, until you figure out the inflight balance point. Then, epoxy them into a place inside the fuse. You would have to drill or cut a hole, then patch it. Works great for in the nose too, especially if you have a tight area. The bb slurry can be molded to fit the area. I have used that technique in cowls too.