RE: adding flaps to an arf
Steph, your not done yet.
Those control horns are … well … junk. The screw is in the middle of the bearing surface (not a good location for positive control) so over time the wood in the attachment point and the plastic in the control horn will deform allowing it to rock. Then your controls start to get mushy. You can put a screw in the front and back of that horn to prevent this but before you do that take a look at my post #17 specifically the placement of the control horn. Notice it is a ½ inch back from the L.E. of the flap that gives more movement of the flap and better control. You can accomplish this by turning your horns so they look backwards.
Joe