There should be a balsa post on the inside of the vertical fin, only the skin should be fiberglass or composite. I would just line up the vertical stab and rudder in the exact position they will be in, tape them in place, measure, mark and cut a normal hinge slot with an exacto knife. I have done this before and its no big deal. Works just like cutting balsa. You just have to make the hinge slot slightly wider and very slightly deeper than the hinge it is going to receive, just like cutting into balsa.

The only sticking points would be to make sure there is no foam behind the hinges, if there is you cant use CA as it will destroy the foam. My bet would be that the fuselage skin at the vertical fin post does nothave any foam in there.

Drill a little hole right on the hingeline with a tiny drillbit, like 1/32 or even 1/64th and see whats in there, then you will know what to do. Im bet that when you pull the bit back you see wood. A little exploration to see what you are working with would be appropriate unless someone can tell you exactly what you have there.