A fin is just a small wing. I'm stealing a picture from one of my wing designs, because it is handy. But the concept and the pattern is the same. Look at the bending moment. Stiffness should follow a similar pattern for best efficiency - and likely for best resistance to flutter.
Just look at the middle of the graph as if it were the body tube, and the left and right edges as if they are the fin tips. See how quickly the bending moment decreases?
So how should the thickness taper?
Stiffness at a chord station is proportional to the cube of the thickness at that station, times the length of the chord. Halfway out you need about 20% of the stiffness. The chord halfway out is a little over half the root chord on a typical fin. Anyway, somewhere between half and full root chord, depending on shape. For example, let's say the tip is a third the chord of the root. So halfway out the chord is 2/3 the root chord. So the thickness needs to provide 20% / 2/3 or 0.3 of the stiffness. The cube root of that gives 2/3 the thickness of the root. That's halfway out.
One could be lazy and just make the tips about 1/3 the thickness of the roots, use a linear thickness taper, and call it a large improvement over the typical constant thickness.
Hopefully one can also see that just changing the layers in T-T only makes a dent in the right direction. It just makes a person put in more layers to help prevent ripping off a suboptimally profiled fin. IMHO, of course!
With the appropriate thickness taper for the profile, then any reinforcement need extend only a little beyond the fillet. Just enough to securely bond the fin and reinforce the fillet. The fiberglass or carbon added on top the fillet then just becomes a structural member.
BTW, different subject, fabric over leading edges. Try Kevlar or layers of light fiberglass, they conform better. The fabric angle for just that part should be at 45 degrees to the leading edge. It will bend better, and all fibers go over the edge.
Prepare material by laying down a sheet of wax paper on your bench, and spraying 3M77 from about two to three feet back, aiming along a line a foot or more above the wax paper. You are making a light fog that will settle on the wax paper. It is a very light mist coat.
Carefully - you get one shot - lay on a section of light fiberglass or light Kevlar. Preferrably on a bias, but most fins are so short one can cut at an angle and still have enough length.
Flip it over, and you can lay out your pieces. You want strips at least half an inch wide. You could go somewhat wider if you want. Cut out with scissors.
How dexterous do you feel?
Fold back a little wax paper at one end.
Spray some 3M77 on the fabric side. Let it fairly well dry.
Put centerline of sticky biased fabric strip along edge of fin. Rip off the wax paper. Ripping it off can work. Pulling slowly never will. Then smooth it down with your fingers, leading edge to trailing edge direction.
The minus of this approach for fins is the 3M77 blocks epoxy uptake.
So for rocket fins, perhaps instead try this:
Lay out a strip of drop cloth. Lay out some fabric on the drop cloth. Wet out the fabric with rollers and epoxy. Dry it down so it doesn't have puddles, and is uniformly coated with no bubbles. Lay another piece of drop cloth on top. Keep a sample of the wetted fabric separate. When the sample is starting to get firm, but is still sticky, roll the drop cloth sandwiched piece and put in freezer to stop the cure.
You've just made some prepreg.
Now do your fin work, and when the fin fabric is still a bit tacky but firming up, pull your prepreg out of the freezer. Cut a strip, pull off plastic from one side, and smooth it down where you want it. Then pull off the other plastic - or just wait until it is more cured or fully cured. The plastic will probably pull off even then.
Hopefully I've given people some ideas.
Gerald
