Not sure if the pic is clear but I put my rudder servos in rear fuze. I mounted them on a board and dropped them in using nylon line through the arm slot in fuze attached to servo horn to fish them through.
Then screwed them down through fuze bottom. Fins was bare wood with film covering. Not too rigid but I never had flutter at 400kph+. used plain old 504's
Also the rudders are only halfway to top of fin.
Andre