Make up template to align your horn at a right angle with the hinge line of the surface, this is to ensure perfect alignement of both sides, with a top hinged surface this is fairly involved, but non the less needs to be done accuratly.

Once your second slot is cut you can put one of the original horns into place against one of your new horns to check the alignement, thats assuming the original slot is in the correct place. If satisfied with the positioning, tape the cover plate into position and cut around the ouside line to remove the plate, this allows you to roughen the area where the cover plate will be glued and to load the slots with microballoon mix without covering the rest of the plane wing or tail with glue.

Mike

EDIT: do remember to roughen the bottom of the horn where it inserts into the wing/tail to ensure a good bond with the microballoon mix.