I think you mean like the flyrace board, with it's adjustable daggerboard fin between the straps?
To get the benefit, the fin has to be pretty big, and adjustable, unless you only want to go upwi you should get intnd in light wind, and have bad turning etc.
Maybe you should check out viewtopic.php?f=1&t=2380246&p=791962&hilit=PBKiteboarding#p791962
and get in touch, these guys have put a flyrace fin on a smaller board as well as the door.
The install would be more involved than what you describe, which is for a normal fin, 70mm or less.
If you just put normal but "too big" fins on a TT you will just make it unwieldy.
If you want to make a fin like the flyrace fin, a thin section of a circle will work, if you can mold that or grind it out of a g10 bar....
Directionals are fun too....