As a KF-III owner I should know this, but, I believe my spar insert is a 3' or 4' extruded aluminum 'I' Beam. Compared to other twin spar tube wings --and my old Avid Mk-IV comes to mind with flat 18" long plywood inserts-- I would have thought any wing "flapping" would be minimal and visible primarily out at the tips a couple/few feet beyond the lift strut attach. For KF-III owners who have experienced 'flapping' on occasion, for reference just how much flex are we talking about here? The comment about outboard flaperon weight position did catch my attention, interesting to note that the Avid weights were toward the middle and I don't recall too many (any?) flutter posts.