As I recall (it's been about 10 years since built) on my old Tiporare with anhedraled stab, I simply joined the sheeted stab halves as usual, then placed them in thru the cutouts I made in the fuselage tail area. I left the balsa tail post uninstalled out of the vertical stab until after mounting the stabs, so I had good access to the inside of the joint. The holes were cut using the root shape of the stab as a template, using a Dremel. Open the holes up a bit as necessary. Rough up the inside of the fiberglass at the glue joint a bit for good adhesion, and wipe down with alcohol. Place the stab in the hole, and tack glue in place and in proper alignment, using small shims as necessary. I then sealed around the outside of the joint with 1/4" wide masking tape for the next step. Epoxy the stab in place from the inside, using 30 minute epoxy with micro balloon filler added (if your holes aren't real precise this helps), and some small strips of lightweight glass cloth laid into the joint, top and bottom. Remove tape and you should have one neatly and strongly mounted stab. Fill the outside of the joint as necessary.
This made a very strong and secure mounting.

Good luck. Hope this helps.

Rick