Go to a fabric store (you know.. where you can buy material for sewing). Get yourself two good cloth measuring tapes. It is stiff enough to use, you can pin one in place on each wing tip at the same point on each side and with the pin through the tape at the same point on each tape. Then pull it over to the tips of the hor stab. Because both are the same, the measurements will be easy to make on each side.

Pin the hor stab in place and mark it with a sharpie marker (permanant but will wipe clean with alcohol after you are done).

Strip off the covering, then epoxy it in place using 20 minute epoxy. After you apply the epoxy, and before it sets up, re-measure it and check that it is level (parallel) with the wings. The laser measurement that Moondoggie said is the best way. Then, tape it secure and make sure it does not move while the epoxy sets up.. don't move it for at least an hour.

CGr.