the numbers that Ola quoted are similar to what I have found ,also the rest of his advise as well.
I'm just starting the tri fin project to a freestyle board.
The mini- tuttle box would be best, it goes thru the hull with 1 bolt. If you use a surf base system, my research found the Probox best suited. Futures next and FCS NO but very popular with surf boards. Their are others as well. Fins pretty much just fit that particular box. quite a range of fins available, unless I used a mini-tuttle or US box I would not go longer than 5" for the 2 fins. About half the advisors I have recommend tying the box system into the underside of the board, I plan to follow the Probox instructions, which don't. The tri fins seem to work better with a swallow tail.
the mini-tuttle is used by some wave directional kiteboards, Best, Slingshot, Cabrinha.
and zero cant .