I usually make the retract cutout in the core first. Glue the retract mount plate into the core before you sheet it. I cut a groove in the core and laminate a balsa spar with carbon fiber tow. Try to extend the retract plate to the spar. Glue all of this in when you apply the sheeting and bag the whole mess. I like to build the servo mount to join the spars together at the wing seam. When you glass the center section, it will tie the load paths together.