Heres what I did:
1.) Dont change anything on the angle of the fin mount on the gearbox. Just make sure the seam down the middle is smooth with the rest of the flat surface.
2.) make you a plate from 1/16 plywood. Make sure that its big enough so you can drill the tailfin mounting bolt holes in it(after its completed) without it interfering with the bolt that secures the gearbox to the shaft and make sure when you put the nuts on that they will clear the gearbox and the head of the tailrotor gearbox bolt.
3.)glue on your your ply tailfin mounting plate with CA glue
4.) take a dremel(or sandpaper by hand) and rough up the plastic gearbox casing .
Get an old tailboom or use the good one rub some candle wax on the end to keep the fiberglass resin from sticking You might want to rub a little wax in the other end where the bearing goes. Slide your gearbox on the boom you can hold the boom while working with on the gearbox.
5.)Cut your fiberglass in small pieces about 1/2" squares and some 1"x 1/2" youre going to layering and overlapping these pieces over your gearbox. (cut enough to do about two or three layers over your gearbox)
6.)Brush resin over your gearbox and plate.
Start layering the cloth over the gearbox(Its a messy job) dab the cloth down in the corners and around the edges with a stiff small bristle brush. keep working it until you cant anymore. Mix up a little more resin and repeat. After you get all your fiberglassing done and its cured, start shaping by sanding. Dont forget to remove the tailboom so you can shape that end also. Put boom back in Gearbox when applying more coats of resin. After getting close to to the final shape, mix some more resin and brush it over the gearbox (dont put anymore cloth on). let it set up and cure.
Sand some more until you are satisfied with how it looks. all thats left is drill out your tailfin mounting bolt holes and carefully redrill the hole that your T/R pitch change bellcrank bolt goes in. and open up the recessed nut socket on bottom of G/B (use a razor knife) and open bolt hole on top of G/B. If you get resin on the inside of gearbox where your gearbox bearings goes, it should chip out pretty easy. Do what you can to not let the resin build up in those places.
The last thing you should do before painting is drill your holes for the bolts that you will be using to secure your tailfins. Make sure your bolts and nuts wont interfere with the gearbox and bolt head

The process actually sounds harder than what it really is. It took me less that two hours to complete and thats including the waiting for the resin to harden.