Extreme Flight 125" EXTRA 300 Assembly Manual page 4

4. Use a sharp hobby blade to remove the covering over the slots at each of the 3 locations
where the dual truss control horns will be installed. Place the horns into the base and
insert into the slots in the aileron. Trace around the base with a fine tipped felt marker.
5. Use a sharp hobby blade to remove the covering 1/16" inside the line you have traced so
that the fiberglass base plate will cover the exposed edges and prevent them from lifting.