Control horn attachment
Even if your holes are straight, they could be off a little, so here's another good tip:
Put the horn on with one screw through the control surface. Start the screw in the backplate, but don't tighten it, just get the screw started. Now put the second screw through. Now you have some leverage to align the screws to the holes in the backplate. tighted each screw a little at a time until both are tight.