I usually drive in the screws on all servos to thread the hole, then back them out and put a drop of thin CA in each hole. That hardens the threads in the balsa, kinda like the threads in a nut. Once the glue is dry I mount the servos. If your concerned with just the area around the opening that the servo goes in you can glue in screw blocks inside the fuse. 1/4" X 3/8" works good. That is what has to be done when installing the pull/pull setup on the rudder. Joe