As another idea, I use lead diver B.B. weights. After you figure out what you need, drill a hole in the bottom of the rudder. Mix a slurry of the bb’s and epoxy. Roll hard paper for a funnel, or squeeze the epoxy mixing cup to get it into the hole. Make it big enough, otherwise it is an aggravation. This way it is hidden. As long as it isn’t ounces and ounces. It will make the monokote bulge. Works best with fiberglass rudders. Then fill the hole with a bolt. Or, tape the hole and let the epoxy harden to seal the hole.