Use a little thin CA to fix your fin, then run light weight weed eater line around the fin, stab and lower fuselage. Drill 1/16 holes in the fin & stab tips and through the lower fuselage (or put a screw in the lower fuselage and wrap the weed eater line around it. Run the line through the holes and secure with medium CA.

Most ARF tails are flimsy to start with and are very susceptible to hangar rash. Weed eater line or heavier bracing rods work wonders.