The correct way is to locate where the screws will be for the trim tab plates, drill past the length of the screw, potentially into the balsa core, hollow out behind the drill hole with a ball end rotary file drill bit, and fill the cavity with thickened epoxy. I also had to drill a small vent hole above to prevent trapped air when filling. Doing this your making a water tight plug behind the fiberglass for the screw to bed into. Screw your hardware into those plugs using some 3m 5200.
I know a lot of shops in the area, but you might consider doing it yourself to ensure its actually done correctly.
That being said, high on my list if you didn't want to to do it yourself would be EQ marine in Cornett Bay, or Bitter End Boatworks in Bellingham. Just be very clear what you want done, because most shops are just going to drill holes, and screw the tabs onto the transom with a dab of 5200. That's all the c-dory factory did after all.