I would like to add some extra fins to my LW Door board as seen on some other models. I'm thinking of 2 7cm fins on heel side just behind the footpads.
Drilling trough the material > filling the side of the hole with epoxy to keep the water away from core and mount the new fins, is that it? I would use plastic washers under the bolts to protect the top sheet.

Would the 2 big fins make any difference? I assume that the side effect could be more drag but upwind performance should improve.

The fins may actually hurt performance, and make the board less fun. With those fins those small waves will not be little sliders any more plus it will make the board stick to the water. Its the boards rail/shape that makes the board ride upwind.

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....

@BWD: Actually the Flyboard 4 gave me the idea which has one fixed symmetrical fin in the middle on the heel side.
The long fin of the Flyrace seems too radical to me and I don't like the idea of constantly adjusting it during sessions.
I will look into that thread anyway, thanks.