Hi, name's Ken (Starfighter), been building/flying for 43yrs.
Answered this in another post a few weeks ago, but nobody commented on my suggestion (pro/con).
This is VERY EASY to do. HOW? Well, all you have to do is use the base plate of the horn as the screw guide.
Yes, use the base plate of the horn as the guide for the screws.
You do this by placing the horn on the surface at the correct location, and then take a pin and puncture the surface at the screw locations in the base of the horn. THEN, use the 9/16"socket head servo screws available from several sources (as they are self tapping, and #2[same as the control horn screws]) as your drill bit. Use these screws to finish drilling the hole through the surface, while the base of the horn serves as the drill(screw) guide. The horn base plate will keep the screws vertical and the correct angular relationship to the control surface, and the exit holes on the other side will line up correctly for the back plate of the control horn. After I've finished drilling(screwing?) the holes, I apply thin CA to harden everything up.
Try this out, it really works!