I did the under seat install. There is a doubler plate to reinforce the bottom skin. I've never noticed any problem(s) nor have I ever heard of any problems with that installation. I have the TT servo...
+1 on what Kyle said. Also I don't think a rib attachment will be strong enough. On my -6, the servo movement is strong and even flexes the skin, thus the doubler is used.
Gary,
Are you getting flex WITH the doubler?
Craig