Mine hasn't warped in the 9 months I have had it, Had some major crashes with it, and the most work the fuse has ever needed was the replacement of the lower spruce spar. And the fuse glued back to the rear part of the wing many times, lol.
This is a heavy way of constructing the fuse, but it sure is strong!!!!!!!!! Not stiff but stays in one peice.

I agree with Paul Swany's comment about the fuse height and perhaps the addition of some CF. The rudder is waaaaaaaaay to small and mixes a bunch in knife edge, it rolls opposite the direction of rudder input.

I have modified my whole rudder and fin arrangement, making the whole thing symmetrical. Ie removed original rudder, added another rudder post on the bottom, and then made a new rudder that is about 180mm long, (about 7 inches)
Much more rudder authority, and no pitch or roll coupling in knife edge, The fuse is still way to small in area, so it wil only slighty climb in knife edge, taking a hell of alot of sky to complete a knife edge loop. The second half is scary!!!!!!!
You need a big servo for this rudder though.